Being French I use a French server, www.nuxo.net
I fear you'll have to make a little bit of search for other provider. Go to www.jabber.org, and search the public server list (www.jabber.org/user/publicservers.shtml). It is not always obvious if those servers offer the gateway to other systems.
Have a look here www.jabber.org/user/userguide/#findgateways, it may help.

Anyway, I recommend you to do the registration / setup on a regular computer, check that everything is up and running well, and then switch to the N800. That's the way I did it and it went pretty well.