Just can add multiple accounts but only one of them can be active at any one time.
To be honest the Jabber support in iChat is a bit poor. I've never been able to figure out how to 'browse' and such like, hopefully it will be improved with Leopard. Psi is worth a look if you're looking for a Jabber client.
From what I understand it can't at the moment support more than one Jabber at the same time.
What clients are you trying to add into Jabber?
I just used the Psi/Google Talk/Jabber trick to apply my icq & msn list into iChat's Jabber account and it works pretty darn good.
But I am unsuccessful at importing the AIM list into the Jabber list so I don't have two buddy lists open. I tried adding the AIM service into Psi, but it won't allow me, does anyone have any ideas? Having two buddy lists is a bit of an eye sore.