OK, I've worked out the answer, elrafo.
You need to call up iChat preferences. I do that by clicking on the iChat docking icon, which makes iChat replace the finder default on the top of the screen pulldown row. From there find and change preferences, marking iChat as the default instead of jabber. You can also add other babble-systems like icq or whatever else you use, but making them the default.
I guess this is very elementary for most Mac users, so perhaps I should have put this question as a thread in the mac basics forum rather than here in mac OS apps...