After some surfing around on the Net, I found this article, which shows a Treo 650 that someone has managed to install Linux on. A Wiki page has sprang up on the subject. There are rumours that Palm is building a Linux-based Treo, and from this article it is certainly possible.
"I've been hacking away trying to get Linux to boot on my Treo. I finally got the kernel up and running, but we're still a few steps away from a command-prompt right now.
Note that the LCD is a bit screwy and isn't aligned to the top-left of the screen - I had to hack the LCD stuff so the kernel didn't try to re-initialize it. I can't figure out the right combination of registers to get it set up properly."