Usually I would write “definitely.”
However, when I get the invitation to my cousin’s wedding, I will put an asterisk in the space and at the bottom write something like:
…absolutely, positively, definitely, surely, sincerely, physically, spiritually, mentally, happily, come hell-or-high-water, wild horses couldn’t keep me away…
We’ll see if she still wants me at her wedding 
Zev Steinhardt
I will (see attached) attend.
*Attached letter
. . .be delighted to depart my humble abode for a few hours on [date of event] to bear witness to your union. I assume, of course, that you will need a sufficient head count which is why you have politely asked for me to reply to this invitation. My party will consist of [quantity]. There are no chickens or vegetables in my party but there are [quantity] who prefer vegetables to chickens.
As I sit and reflect upon how happy you are it brings a tear to my eye. I know that eternal love is the most powerful feeling in the universe. I know that your love will only bloom brighter and bigger in this world’s garden. I feel so insignificant and yet blessed to be acknowledged by your thoughts. To not attend would be sacrilege on our friendship.
In short, thank you for allowing me to . . .
So nice to see someone else who reads Miss Manners. I must confess that I always respond on my personal stationery, using this formula, rather than send back the dopey little card. I hate 'em.
To decline an invitation, substitute “regret that they are unable to accept” in the second line.
…and say “the very kind” rather than “the kind” invitation.
Winky, you nailed it! That was what I was looking for… yesterday.
However, after thinking about Sunshine’s post for a bit, I’m thinking perhaps I might just take the minimal route and leave the space blank.
I don’t know, I’m gonna sleep on it for a few days.
(and please don’t point out the obvious “you’re giving this way way way too much thought”. This I know.)
Do you like me?
[ ] [ ]
Yes No
What? Quit looking at me funny.
There’s a wealth of formal answers at eHow, but I like their step 5:
Consider having cards printed with your name and address for cases just like this.
Print up your own cards with whatever smartaleck answer you choose. These invites never come alone – they tend to come in groups. Having your own stock of reply cards could save time.
If the person reading your reply doesn’t really know you (i.e. you’re a friend of the groom), then you could get away with replying in Mandarin, with an ambiguous broken English translation:
I humbly to wish invite OK.
That should cause at least a second glance, and possibly stir up a whirlwind of activity.
[soapbox]
Don’t cave to etiquette!! Fight the power of conformity. Keep this world interesting for our grand kids and our grand kid’s grand kids and our grand kid’s grand kid’s grand kids and our grand…oh, you get the idea.
[/soapbox]
Fill in the blank with the word “attend” and then cross out theirs.
Malaka, not bad, not bad at all.
And Winky, I’ve tried non-conformity but it gives me gas.
For a minimalist approach, how about: I will not not attend.
Granted, your going against the conventions of the English language, but think of the headache you’ll be giving the bride when she tries to decide what you mean.
[BTW this post is completely off-topic]
Ok, the subject should have said:
Flatulence is natures way of saying, "You stink"
Alright, return to the OP.