It's 'sort of' possible to do it using iPad mail. Any Groups in your iPad Contacts list (sync'd using iTunes or MobileMe etc) will appear when you go to the iPad Contacts app. You can't creat new groups in the iPad app, though.
In iPad mail, when you create a new email, you'll see a '+' symbol at the right hand side of the 'To:', 'cc' etc address box. If you click that symbol, it pops up your Contacts list - and Groups can be selected (if you're not already seeing them) by selecting the small 'Groups' button at the top left-hand corner of the pop-up window. OK - that was the good news!!
Trouble is, you can only select addresses from a Group entry one at a time. So, suppose you had a Group called 'Family', in which were 'Mum', 'Dad', 'Sister' etc. In the Groups window that appeared after you pressed the '+' in iPad mail, you'll see those entries, but you can't select them all (at least, I've been unable to do this!). So you can touch 'Mum', and Mum's email address gets entered into the 'To' field of your new email. Then you have to touch the '+' symbol again and then touch 'Dad' and Dad's email address gets entered - and so on. It's not too bad for small groups, like Family perhaps, but if it was a class list this clearly wouldn't work.
It does have some advantages - perhaps you want to send to most - but not all - members of a Group. The iPad 'way' let's you select Group members one at a time and miss out those that you don't want to receive your email. But, generally speaking, you *don't* want to do that and I can't (yet) find a way of selecting all Group members.
As the other respondant pointed out, there are numerous third-party apps out there that address this problem in one way or another, but it would be nice to be able to do this from *within* iPad mail.
Tim
Scotland