I have not done the HD upgrade, but I think you can do it yourself without much issue (There's no way you're still under warranty!).
Edit - Try Going here.
Pismo parts are avaliable here. I have used these guys to replace the DVD drive, and they are pretty good, even shipping internationally.
The biggest HD on the site is 60GB (5400 RPM) and is reduced to $109 at the moment! - Deal!
Like I say, I haven't upgraded YET, but have often thought of it. As i understand, you only need 1 tool (a torx screwdriver) to do all upgrades of the Pismo. (You could buy some bigger RAM chips to go in the lower slot while you're at it too!)
Hope this helped! I love my Pismo too - makes you feel a little bit superior to all those guys who have the "wannabe" AluBooks!?