If you are comfortable with the kernel, there's info on how to get your own build environment going at:
http://www.androidonhtc.com/wiki/Get_Involved
Then it's time to just start poring over the code in arch/arm/mach-msm/ and looking at the git logs to see what's getting worked on. To my eye, the two most glaring missing pieces right now are the device speaker and (passable) battery status. The battery stuff has come along a great deal in the last couple weeks and will probably take some more steps soon. Audio has gotten no attention.
Start looking at code, start lurking at #htc-linux on freenode, and wait for the clouds to part. That's how to help if you're not afraid of C.
If you are afraid of C, +1 to helping out with the wiki, making the project clearer for people who want to try it out (and may in turn want to get involved with helping themselves).