A short post: just to recommend the Nikon D40/D40x and D90 guides for the iPod touch that Ken Rockwell has produced (I have the iPod, but they also work on the iPhone) – they are excellent guides to both cameras. I had downloaded the D90 guide yesterday before going to a meeting, and in the hour I was on the train I played with the new camera whilst going through his guide on my iPod, and learnt so much about the camera and the various settings. I also bought the D40 guide a while ago, and although at that stage I had been using the D40 for a while, I still found it useful. I highly recommend both guides!
Links are: D40, D90. If you don’t have an iPod touch (or iPhone), you can read the full details on his site, and buy the same content as printable PDF files – follow the links to his home page, click on ‘Nikon’ and a list of cameras will appear, with summaries/guides for each one. (By the way, the only iPod touch guides are for these two cameras, he hasn’t produced any others.)