Most of us, I know I have, have found ourselves in the aggonizing social situation when we have been called on to say grace. What DO you say so as not to offend but also to let them know that you really, really, really don’t believe that God had anything to do with getting that food on the table?
"To whom it may concern:
While I personally do not believe in the existence of any supernatural entities, there are many at this table who do. On their behalf, I thank any such beings that may be listening for their help in making this meal possible, if they in fact did so. Amen."
OK, that’ll prolly piss 'em off too. Basically, I think in this situation you’re screwed. As a fellow atheist, I have always just said the grace I learned in childhood: “God is good, God is great, let us thank him for our food. Amen.” It’s fast and easy and you can say it by rote without thinkng about it much. My folks know I’m an atheist and don’t ask me to say grace nowadays, but I’d prolly fall back on the old childhood prayer if asked. I’m not gonna change their minds and they’re not gonna change mine.
I’ve always just simply said, “We give thanks for this food and for this gathering of friends [and/or family] and for all of the blessings in our lives.”
No need to be confrontational with your disbelief. The “prayer” I utilize doesn’t specify whom I’m thanking. It could be fate, or karma, or the Big Guy upstairs. I don’t consider the word “blessings” to be religious in nature, but religious people can take it as such, so everyone is happy.
I smile and decline. Thanking it’s a foolish exercise is no reason to insult those who put stock in it.
Well, I wouldn’t say, “Rub-a-dub-dub, thanks for the grub.” No matter how tempting…
I usually say the standard, “Bless this food to thy use…” But in your case might I suggest, “Thank you for bringing together these friends [and/or family] for this wonderful meal. Despite the fact that we might not share similar religious beliefs, we are all grateful to be here and grateful for this food. Amen.”
I’d add the Amen at the end, both as a sign of respect to those who are religious, and as a way to signify you’re ready to chow down. Also, it might just confuse 'em. 
(Btw, welcome to the boards. I like your username.)
To Evil Captor: Good suggestion and response. As an aside let me state that I am not an Atheist. Not in the strict sense anyway. I don’t believe in organized religion or in an anthropomorphic Sky God (to borrow from Gore Vidal). I am a very religious person in my own way. I believe there’s something out there. I just don’t know what.
Thanks for the response…
Thanks, Leander. I do love to read.
As for your suggested prayer(?), I think it’s great. Sounds like something out the American Humanist Association pamphlet dealing with this issue and other difficult social situations.
To get off the subject a moment. I am a new member. I keep having to log in over and over. Is this this SOP or am I doing something wrong? 
“Let us be grateful for this food and these friends (or relatives.”)
The “Let us be grateful” part will satisfy the believers and you, each infering as they wish. Plus the lack of a contraction will make it sound Biblical.
Example for AvidReader, “Let us be grateful for this cookie.”
Why make a deal out of it at all? Just be courteous. Think of saying grace as just a social convention, like shaking hands. You don’t have to thank God.
Just say, “Thank you for this food, and for this peaceful time of friendship” or some other vague thanks. And from your standpoint, you could be thanking your host. Because that’s what you are doing. Indulging someone’s belief while sharing their food is just a neighborly thing to do.
You don’t always have to be taking philosophical stands.
“I am grateful for this moment, this company, and this meal we are about to share, and for the opportunity to speak this prayer on my friend’s behalf.”
Pause
“Can I have an Amen?”
Tris
I think I would politely say that I’m not religious. I don’t think it would be honest to pretend to be praying to a God I don’t believe in.
Try looking at this:
PLEASE READ - for those who are being logged out unexpectedly
I’m not a religious person, but I am an honourable one, and guests should honour their hosts.
If the host wishes to say grace with you, really, why the fuss? You think saying those words will cast a magic spell on you? 
“I would like to thank {insert name of host here} for providing this scrumptious meal, for I’m sure he/she had more to do with it than some postulated supernatural force.”
Sure to generate a lively conversation. Or stunned silence. Either one works for me. And I’m not likely to be asked again.
If the host is Wiccan, possibly! This, by the way, is from someone who used to dine with Wiccans regularly until they moved 600 miles away.
AvidReader, welcome to the Board! As the most devout person in my family, I usually get asked to say grace on special occaisions, but even so, I usually just thank God for the food, company, and other assorted blessings. If I were agnostic or atheist, I would probably say something gentle and discrete about how grace isn’t part of my tradition and suggest the host might want to ask someone better suited.
I, Brian, I welcome you to the Board, but, as the descendent of a long line of Lancashire peasants, I have to tell you I don’t believe in Yorkshiremen!
CJ Howorth
I either read from the book, or just say Baruch ata Adonai, eloheinu melech ha’olam, hamotzi lechem min ha’aretz.
Hey, you asked.
“Bless us, oh Lord, and these thy gifts, which we are about to receive from thy bounty, thru Christ our Lord. Amen.”
Regards,
Shodan
All you have to do is say “Good wine, good meat. Good God- let’s eat!” once and they never, ever ask you again.
At which point I get up and leave since scripture commands I eat only what is dedicated to God.
Is it necessary at that point to let them really, really, really know that you don’t believe that? Can’t that wait until after people have started eating? (which is not to say keep silent on the subject, but can’t it be moved 15 minutes into the future so the food won’t get cold?)
The words “Thank you, but why don’t we let Joe do the honor tonight.” seem sufficient.