maxppc, do you actually mean jabber-server as in jabber.org - or just any XMPP-server out there?

assuming you're talking about XMPP in general:
while I do not notice this myself (=on my own N900), I see people who use the N900 dis- and re-appear as you describe it when I'm on my desktop computer. I get the signal that a person signed off, immediately followed by "signed on" again.

I always thought this is because these people are on the move and maybe 3G connection drops every now and then, but maybe it's something else...