Telepathy allows various protocols to be used and added to the N900's messaging. Telepathy-haze allows the use of Pidgin protocol plugins allowing MSN, AIM, ICQ, Yahoo and others. There are also other plugins available such as telepathy-butterfly, which is an alternative MSN implementation.

These plugins are already available for the N900 and working (though still in a beta state). Yes, they are already integrated, but have to be installed separately. Most major IM platforms will be supported.

The N900 is an always on machine. Therefore these IM protocols are also going to be always on unless you decide otherwise.

Hope this helps.