I finally found a free instant messenger client that talks Jabber: Talkonaut (via e61iblog), after trying out various things including mig33 and EQO, which promised IM (as a side effect of cheap/free voip calls) but didn’t deliver what I wanted.
Unlike these others, Talkonaut was easy and quick to download, and didn’t require registration for any other service I don’t want. It worked fine chatting to a couple of people on gmail, but did seem to seem to lose the wifi connection after a while. I haven’t tried it via GPRS yet, or the very occasional 3G signal I get.
It’s Java, so should work on lots of phones.