I too find a good serif font to be easier on the eyes during sustained reading. I've tried quite a few after running them through the PalmFiction font converter, but my current favorite is 14-pt Cambria, one of the new fonts that came with Vista. It's very legible (for me, anyway) at about 135 words per page on my 480 x 320-pixil screen (ca. 4" diagonal).
PS I found an interesting discussion of this topic at
http://www.alexpoole.info/academic/l...urereview.html