So far I only support Voicestream, but I coded it extremely modularly so I can very easily support other providers ... just send me information about how your cellphone/pager system works. I was going to add Nextel support in (because whitaker uses Nextel and I know his text messaging email address) but I want somebody from Nextel to tell me the limits on the message size first. LiveJournal will validate that your message fits on the remote user's display before sending it.
On the userinfo page there's now a link to send text messages .... check out mine for an example. Send me a message if you like (I have it set at friends only, though).
By the way, this is a paid-user only feature ... to be a recipient of the messages, that is. Any user can send a text message without being a paid user. Hopefully I won't piss too many people off by that, but if you're pissed ... well, you should've got a paid account. I figure if you can afford a cellphone you can afford $25/year for LiveJournal. Please, no bitching. :-)
You can read the new FAQ question if you like --- How does the text messaging feature work?
It was fun developing this --- whenever I tested it my monitors would start shaking for a second before my phone beeped. Damn radiation... cellphones are going to give us all cancer. :-)