I'm using an HTC Touch Pro2 which runs 800x480 resolution and I use Freda software to read ePub books and Mobipocket to read .mobi and non DRM amazon books. I love it. My mom is also using the same setup.
Both the HTC HD2 (Windows Mobile) and the EVO (android) have the same resolution though a slightly larger screen. I currently use the winmo simply because I've been on that platform for years, but considering the future of the 6.x platform is limited I'd strongly recomend going with the android EVO since you'd have to switch platforms anyhow. I'd steer away from the palm simply because of it's small screen.
Both the Winmo and Android have a good selection of software, though if you'd like to purchase either Amazon or B&N books you'd almost have to go with the Android. It's unlikely that any future software will be developed for the current winmo platform and the future winmo 7 support is totally up in the air despite what bill gates would have you believe. Android software is very actively being developed these days.
My next phone to be purchased in a few months will definitely be the EVO. Personally I prefer using my phone to read my books. A Nook is just to large to carry in my pocket.
