This could be a killer new feature for some people. Apparently Google at least partially pays for it by the return messages from the mobile device (that you’ve first sent a message from Gmail).
The way it works is that you start with 50 credits (displayed in the Gmail chat window where you send the initial SMS message). Every SMS message you send from Gmail uses one credit. Every message replied to from a mobile device restores 5 credits (for a maximum of 50). If you use all the credits, 1 credit will be restored automatically in 24 hours. So I guess you could game the system (because you’re a rebel screwing the system, man) by never replying from a mobile device.
So I’ve been trying it with both supported carriers (DTAC and True) in Thailand this morning, and there’s one big problem: The messages are arriving to the phones 8 to 10 minutes after I send them from Gmail. Too slow. The return messages are much faster, taking 2 or 3 minutes, but this is also very slow compared to sending between mobile devices in-country. I hope Google can use their magic influence to make their free SMS messaging a bit more usable in Thailand, but maybe it’s a case of you get what you pay for.