Zefanja, a member of the German Palm forum Nexave.de, has published a blog entry where he posted some screenshots of his Treo 650 running OpenMoko, a mobile Linux GUI.
This has also been tried in July 2007 on another Treo 650, and it also runs on the Palm TX. Among this GUI, there are also some others, including GPE and OPIE. These two also run on almost all Palm OS 5 PDAs and users report that they are much faster than OpenMoko.
You can find information about OpenMoko on their project site.