Now that xmms2 is usable development on the xmms-jack plugin has ceased. xmms2 has a native jack output plugin and was designed with callback based low latency audio systems, like jack, in mind.
After a handful of hours of hacking I've got the xmms-jack plugin working under xmms2. Should have the first batch of changes in soon so expect full xmms2 support well before xmms2 is officially released.
Big improvements in this release: Resampling and persistent connections.
Should be quite usable now after fixing the pause at the end of some mp3s bug.
New driver available, loads of fixes and compatible with the latest release of jack.
Uploaded the latest release version.