There's a really great review of the new iBird Explorer at Cloudy Days and Netbook Nights (done by my buddy Steve the Zeiss rep). The iBird is put out by the folks over at WhatBird.com. The interface on it is really, really cool and very intuitive to use. I wasn't thrilled with all of the illustrations on it and was hoping to hold out for Sibley or National Geographic to put their guide on an iPhone, but reading Steve's review I'm a bit more excited about it.
The creator, Mitch Waite is dedicated to constantly improving the product and not only adding illustrations but also photos of each species--that suddenly makes this much more interesting to a hardcore bird like myself.
For an in depth and excellent review, do check out Steve's write up.