Hmm... Thanks.
At a previous job (years ago), we used AIM to communicate between offices, but we could add mobile numbers to our contact lists (+15551234567). I could send messages from AIM to others' phones and they could respond to those messages by sending SMS.
So, it sounds as if you're saying that this is no longer possible, and there is no way to create an AIM account, for example, add contacts' mobile numbers to my buddy list, and then use those contacts to send/receive IMs from my phone. Do I understand correctly?

**I've done more searching online and everything seems to suggest that this is possible, but I haven't found anything specific to Maemo or the N900. Trying to search for " IM " is painful, especially when many search tools ignore 2-character strings and the rest just return thousands of records containing " I'm ". And it's not as if anyone types "instant message" anymore.

For every semi-recent online post stating that adding phone numbers to an IM buddy list is possible, I find another claiming that it isn't, or that it was but is no longer, or that it wasn't but now IS possible.