AT&T is a straight GSM network. Their phones are completely incompatible with the Sprint/Boost CDMA network. You could probably get it hooked up with the Wal-Mart T-Mobile Friends and Family service though.
Not really... Most of the Sprint phones for Boost that you see on Ebay and other sites weren't cleanly flashed (clean E.S.N.'s, etc...) Technically, Boost doesn't allow flashed phones on their service, so it's highly unrecommended. Having a phone "legally" flashed to another carrier such as Metro or Cricket isn't always foolproof that all of the new carrier's services such as S.M.S., mobile internet, or M.M.S. will work on the phone either
Too much headache. Not worth the trouble unless you find a forum that'll cater to the problems and suggest solutions. Since it's illegal that's kinda iffy too. You'll constantly run into problems and have to look into solutions. Just wait a bit longer and Boost will offer a fancy 4g phone soon. From what I hear/read July 2012.
If that's too long of a wait then try Straight Talk. From your previous post it seems that you already have a AT&T phone. Ask for a AT&T sim from Straight Talk and put it in and go. $45 a month, unlimited. Their network also has a larger footprint than Sprint/Boost.