Much of the work done to make the Linux Kernel more suitable for real-time audio work is steadily being merged into the official kernel. Right now, the 2.6.30 kernel in Testing already performs impressively.
Some linux audio users, however, might desire to squeeze some more performance out of their setup by modifying the rtprio's of individual IRQ handler threads. For this, CONFIG_PREEMPT_RT is needed, which is not in the kernel yet.
There is an ITP for this, but it is a complicated challenge. We might want to try to document how an end-user might obtain a kernel with CONFIG_PREEMPT_RT enabled here.
The 64studio guys seem to have built a recent one for Ubuntu Jaunty and put it up at http://apt.64studio.com/backports/pool/main/l/linux-source-2.6.31-rt11-686-multimedia - seems to be fully functional at first glance. Note that there are known incompatibilities with nvidea video card drivers. Benchmarking with http://github.com/koppi/alsa-midi-latency-test/ , the stock Debian kernel in Testing actually outperforms linux-source-2.6.31-rt11-686-multimedia, even when increasing the rtprio of the IRQ handler thread the USB audio device was on.