Ok, it is clear that you can't trust incoming or outgoing international SMS to Sprint, since they only interact with a limited number of carriers and they don't even give you an error message letting you know that they don't support that carrier.

It's not reasonable to know every carrier's sms email address (eg, @messaging.sprintpcs.com)

So there would be three likely solutions.
1) web-based through Blazer
- pswin.com - seems to be for heavy or corporate use
- skype - can only access through a skype client that doesn't work on palm

2) a palm or java app (maybe bundled with an IM client);
- EOQ - the company doesn't recommend using on a palm due to palm's poor implementation of Java. Don't know if this is just a voice issue or also affects SMS
- IM+ - doesn't seem to do SMS

3) a email based service (likle teleflip for domestic) that forwards into the SMS gateways
- pswin.com - seems to be for heavy or corporate use

Anyone have a better idea?

The only solution that seems to be ok is ipipi.com, though it costs 10c-15c per message with $7.50 minimum and involves manging another account.