Why should I use a phone-specific proprietary application with a proprietary protocol and annual fees, not delivered with any OS/device, when we already have built-in support for standards-based protocols supported by multiple platforms (XMPP) in use by large companies and sites (Facebook, Google), and supported natively by a number of OSs and devices.

For Blackberry users, maybe Googletalk is the best option, however it doesn't seem to have encryption support (but, neither does telepathy).

For other phones without any XMPP support, maybe talkonout

Have a look at the availability of XMPP-only clients and look at how many multi-protocol IM clients support XMPP.

Now, maybe some features are missing, but it would be *much* more worthwhile to look at getting better features (e.g. file transfer, encryption - all available on other XMPP clients) into telepathy on N900 than having *one* additional proprietary chat application/protocol supported. In the meantime, you can use pidgin on your N900 for some features.

BTW., I have been using Jabber on my mobile phone since 2004, on a Nokia 6600 with MobileMessenger over GPRS.

If you're paranoid, run your own jabber server, and only allow TLS-encrypted connections to it.