I doubt very much you'll be able to use the iBooks app just because you purchased the iPad in the US. You will need an iTunes account to make purchases and when it see's you have a UK iTunes account it'll just kick up an error I expect.
Also bare in mind if you buy in Orlando you'll be subject to FL TAX 6.5%, not as much as UK no, but then theoretically you should declare the purchase re-entering the UK (purchases over £390) and then have to pay VAT on top of that. If you don't declare it and get caught walking through nothing to declare you'll have to pay the VAT and whatever fine they give you.
You'll also only get a US Mains charger, not a major issue I know but just thought I'd throw it in there.
I too am considering buying mine in the US. Undecided at the moment - will have to work with some figures.
Make sure before you fly out to Orlando to check in Dixon's TAX free in departures, if they're stocking them there by then, you can ask them to ship it to your home address after you've returned (for only £6), this way you (totally legally) avoid paying VAT
This usually works out cheaper than buying it in the US once you take into account their sales taxes! (Can you tell I travel a lot? lol)
Hope this helps
Colin