I bought one of the £29 Nooks and rooted it (NookManager). I am not convinced yet how useful it is. Sudoku is good on it, but the display refresh is a bit slow so not sure how good it will be for apps in general. Interesting learning experience though.
For anyone else - the firmware update to version 1.2.1 that is recommended only works if you have the UK 1.2.1 firmware. I accidentally tried to use the US firmware which my Nook ignored (thankfully).
I feel a Raspberry Pi for the same sort of money would be more useful?