I currently Don't own the N900, but I had the N810 and using the default chat, IM app doesn't work if you are behind a nework proxy. I was in the maemo irc to confirm if this bug has been fixed, unfortunately it hasn't. Hence as it stands its impossible to use gtalk or any jabber based protocol with the awesome default IM app for the N900. How many protocols are affected by this bug I have no idea (Perhaps someone with a device can confirm this) I am also not aware if this affects skype on the phone (skype works very well on the N810 even behind network proxies).

If like me you are behind a firewall proxy, or you connect to the web via an http_proxy. and you plan on getting this device. All hope is not lost you can

- Use pidgin which has excellent proxy support ( But lacks the integration of the Inbuilt IM Chat)

- Use your mobile data connection for IMing

-Vote for this bug to be fixed https://bugs.maemo.org/show_bug.cgi?id=718

One of the killer feature of the N900 is the way it integrate all contacts (gmail, skype etc) Into one contact list. I don't know if this bug affects its ability to do this.

NB This bug is more of a telepathy (Telepathy is the backend framework used by the IM Chat app of the N900) than a maemo bug. The bug exist even on the latest Ubuntu karmic