PC to Mac is pretty simple. For a few days things are a bit wierd, for a couple of weeks you can't find everything, then it becomes second nature. Ease of use, configuration, security etc. are all very easy.
As for the iBook, i'd go for a Powerbook if you can - they're a bit better specified - but they are also a fair bit more money. My preference is a 12" powerbook plus a decent TFT flatscreen to plug it into back home, but i'm not using it on its own that much; if you'll be away from home a lot then the 15" (iBook or Powerbook) makes sense.
Also, Apple have a pretty good - and very flexible - Education discount. It's worth finding someone who qualifies, but (alledgedly) nobody checks what you put...
