The 2.6.29.1 kernel is new to Ubuntu 9.04. Combined with the real-time JACK, you get excellent low-latency performance. (Incidentally, I’ve found that also applies to Windows software running in Linux under the Windows compatibility environment WINE – I’ve had success with NI’s Reaktor. Not officially supported, but quite a lot of fun. Ronald reports at least one user running Guitar Rig 2, as well.)