Ed,
My son Andrew is graduating from HS also and will be heading up to KU this fall. He also wanted a mac laptop but we chose to buy him a Dell 1520 laptop with Windows XP Pro and Office 2003 loaded on it for several reasons.

1) Mac's and most of the software for them cost quite a bit more than a comparable windows machine.
2) Some classes, particularly in the IT field, require a student to have a machine that will run a Windows application which you can do with a Mac but you have to obtain a copy of a Windows ($$$) to install on your Mac using Boot Camp to run the app you need for your class.
3) I know very little about them and wouldn't be able to help him much if he has a problem with it. I know Windows pretty much inside and can remote into his machine from home to help him out.
4) There is a learning curve when moving to a new OS. My son is going to have his hands full keeping up with his classes and doesn't need to worry about learning how to use a new OS and it's apps. His desktop at home has WinXP and Office 2003 on it so he knows how to use it already. He can buy himself a mac after he graduates from college.

I don't know where your grand daughter is headed to school at but check with the IT department of whatever university it is for their recommendations. I know at KU they recommend against getting a machine with Vista and Office 2007 on it due to compatibility issues with it and some of their online apps which is why we bought the older model Dell with XP. I wouldn't buy her the mac air; it looks too fragile and it doesn't have an external optical drive.

Cheers,
Bill