Google voice does work. You can get international SIM's from AT&T too I believe. Not sure if they offer them for that amount of time, or what they cost, but i bet that would work. You do realize that even if YOU have international texting, the person you are texting most likely will not...
I'm unsure how it works elsewhere but I regularly send texts back and forth to my brother in the UK from Australia. I just get charged 50c for each message which comes out of my cap credit. You might be able to send them straight from your prepaid sim.
We don't have google voice here so I'm not familiar with it but I do also use Fring with gchat to send and recieve messages with some people via my iphone. It isn't as clean as the iphone sms interface and the other person needs a way to connect to gchat on their phone or be at a computer but it works well enough.
ok, i would suggest that your best bet is to buy a local sim with a data plan, provided that your iphone is unlocked. Then Use a service like whatsapp to send messages to other iphone, Android, symbian phone.
Otherwise the next best bet would be to your textfree.
You can send free text messages to cell phones using an AIM account and people can reply back. This can be done over wifi or cell data service. I use trillian for this on my phone, but the official AIM app, IM+, or beehive should work as well.
if your jailbroken you can use BiteSMS for international texting. Its ~$7 for a licence now and i think you have to buy credits for texting over there network separately but its cheaper rates then a carrier would charge. Check out their site.