grog: as far as I'm aware, you can't get data add-on with prepaid. Otherwise I would have gone that route too, since I almost never use the mobile phone.
I haven't tried it yet, but couldn't Tweakr be used for that purpose (selective notification)?
What about having your servers send you messages via e-mail, Jabber or GTalk? Then you could have your N900 alert you when one of those comes in, instead of SMS. The best thing about the N900 is that it can be on the 'Net 24/7.