IBM has posted an article on its developer site about running Linux on an iPAQ.
I have had my iPaq, a 3835 model that I picked up cheap in an online auction, running Linux for a couple of years now. My installation is now a bit out of date, but it runs happily with my Linksys compact flash Wi-Fi card in the sleeve. It is a little bulky by comparison to the newer models (mostly because of the need for the sleeve to get the CF slot).
If you have an iPaq that you no longer use on a daily basis, either because you have moved on from the whole PDA scene, or simply because you have upgraded to a newer model, running Linux on them can be a fun experiment. Not something for the novice yet though.
If you want more information, check out the excellent resources at handhelds.org. You will find all the software you need there to save your PocketPC installation and install Linux, as well as detailed instructions for every supported model.