Theo,
For as far as I know, the SOAP api is compatible with Jira 4.0 and up.
But I never used a 3.x release, so never had to test that, sorry.
Jira 4.2 should be fine, we're upgrading next week, and have been
testing it without any problems so far.
HTH
P.s.: Most functionality was done by Mario (and his team, AFAIK)