I don't see why not*, colinux is bog-standard linux once you ignore the whole cooperative thing
, but I prefer dual booting. Actually, I only boot up to WinXP if I need to use Adobe Digital Editions, which is rare because I finally got it to play nice with WINE. I think it has been months since I booted Windows.
For all intents and purposes, linux is my only OS.
* -- keep in mind you do still have to run an up to date linux. calibre on linux doesn't have many dependencies, but it does require a reasonably new glibc. Horribly outdated ubuntu remixes might not have that.
colinux should be able to run newer linuxes though...