I recently upgraded my Ubuntu install and just noticed Empathy's "People Nearby" feature. It is not a particularly new technology (it is the Bonjour a.k.a. Salut protocol), but it is packaged nicely in Ubuntu.

Essentially, it creates a local-network IM that requires no outside server connection. You are able to see and initiate IM conversations with other users on your local network that do not require any connection to outside servers. (It is presumably more secure than other standard IM protocols because the messages do not leave the local network, but nothing is encrypted so it is still visible to anyone on the network.)

If you have the Extra Protocol Plugins for Conversations and Contacts installed, you can connect to this local IM network on the N900 by creating a Salut account. This should also find anyone using the Bonjour protocol on windows or mac.

Not groundbreaking or earthshattering info, I know, but I always like it when my toys work together.