I was also thinking about applying to work at an Apple store from when I go down to uni, although I'm thinking that despite my enthusiasm and experience using Macs the fact that I don't own one and won't until I go down to uni. I could use this to my advantage: 'Switched two weeks ago. Oh my, should you do the same! Come this way...'
In terms of what to wear isn't formal always the best bet, although the Steve Jobs way sounds like the way to go. Don't forget those New Balances.