I don’t think I have any new suggestions, just an “oh yeah! seconding” or such or more region specific info to clarify or build on other’s suggestions.
There are lots of places that will take your memory card and burn a disc for you, but some will want you to leave it for an hour or so, so depending on your travel schedule and companions, this might not work for you. It was about US$8-10 last time.
Also, there are internet cafes in lots of places and a lot of those were satellite internet it seems, so you might be able to upload the best images directly to flickr.
Mail is really slow. We always send postcards but they usually don’t get there until well after we get home. So remember to buy one of the cheap phonecards (you can even buy them at the bank) and call home frequently. Our AT&T cell phones got reception and made calls, but it was $1something a minute so we used it sparingly and I’d do anything to change that. My mother died while we were traveling and it kind of sucked to get all the postcards I’d sent full of love and “we have to come back here with you, you would love this place!” after she was gone and we were cleaning out her home. So yeah, send postcards because who doesn’t love mail? But don’t forget to call your parents. (Your particular emotional baggage may vary.;))
The really thin light cord they have at REI in the climbing section is awesome for clothesline and great to have, but only really if you’re camping will you be able to have the time or space to hang stuff up. Otherwise, the lavanderias which are cheap and plentiful have your back–plus, sometimes it’s so humid stuff just doesn’t dry anyway. But again, you’ve got to be able to drop your stuff off in the morning and pick it up in the afternoon. If you are on a really rigid whirlwind schedule, that might be hard. (And, maybe, not very fun.)
Speaking of, I second the poncho/anorak thing. I missed mine. It’s warm down there, but getting wet when it takes forever to dry can be a bummer. If you bring disposable, pack a spare, some of those seem to rip with little provocation. The nice thing about those bigass things is you can usually fit them over your bag too.
And a headlamp is really nice. The Tikka is good–strong and light and water resistant.
And a skirt that goes below the knees is a wondrous thing to have, especially since you’ll probably be coming in and out of the water and clothing changes, or stuck on the muddy side of the road (the road coming from the Belize border into Guatemala on the way to Tikal isn’t paved for the first two or so miles into the country and we were stuck for maybe six + hours in the mud before the “big” hill) and it’s nice to be able to squat with ready made privacy. But I just have those silk wrap skirts you can buy at the farmer’s market, like these (huh, never thought of wearing it in “15 DIFFERENT WAYS!” ZOMG! :). Anyway, they’re light, cheap and dry really fast.
And it is true about the shorts and such being maybe uncool, although if you are staying in hostels the place is probably “cosmopolitan” enough that they won’t bat an eye at shorts, sandals and tank top on the street. You’re another tourist and don’t know any better. 
In a church, that’s different. (Don’t take pictures of/inside churches unless given permission–this was just a big thing in Chiapas, though, I don’t remember it being a big deal elsewhere, and the guys with the sticks WILL hit you. That’s what they’re there for.) There are some nice skirts and cloth appropriate to the purpose available in markets too, that we saw anyway. They make nice souvenirs too.
Have you got your vaccs? We got hep & tetanus sticks, some anti-malaria pills and (?) doxycycline. (The trip meds are blurring together. I remember big white pills that we had to take before and after and some daily pills. A travel clinic from the phone book can help.) Antibiotics are cheaper there. You can just walk into a pharmacy and ask for amoxicilina or whatever, no 'scrip needed. The pepto bismol tablets are awesome–they’re lighter than the liquid and a good preventative.
Ice in restaurants and such are usually from a machine with a filter attached, and a potable water source anyway. We had lots of ice and no problems. Be diligent in handwashing and sanitation though.
So where all are you going? Are you traveling with a group? Flexible itinerary or pretty rigid? Sailing then backpacking! I want to see the pictures when you come back! 
I’d love to go back and spend more time–one of the places we’d like to go back to was Finca Ixobel in Poptun. We stayed in the original treehouse, and they had the coldest Gallos of the trip.