if it's a full migration, check out www.altiris.com we use their migration suite and it is great. It's saved us tons of work. About six months ago we migrated 200 users in about an hour without touching their machines. It was smooth.
They have a bunch of multimedia demos up on their website that explain migration and it's steps. Even if you want to do all the work manually the demos will give you a good idea about what a migration will entail and you will look smart even if you don't feel it ;-)
Don't sweat it too much, one thing I do is waste a lot of time worrying about possible problems, then wind up facing totally different ones, so I worry twice as much as I should. Just worry about a problem when it's sitting in front of you, you just work through it and on you go.
Matt
crashburnrepeat