It was my understanding that the Verizon galaxy tab is on sale for $499 ($100 off of the $599 price), not $400. The other ones (T-Mobile or Sprint) is available for $400 (actually $399) with a 2-year contract.
Anyway, I'm not sure if BB will allow you to use a coupon on top of a discounted sale price.
I hope you're right. Let us know.
Update: I just checked BB's most recent weekly ad circular. It does show both the Verizon and Sprint galaxy tabs are $399 "with new month-to-month activation". Best Buy coupons specify "no-contract phones". It's questionable whether this is considered "no contract", or a "phone". Additionally, from what I've read online from tech sites, you must sign up for at least the first month of service, and there will be a $35 activation fee. And if you end the 3G service and decide at a later date to reactivate it, you'll have to pay the activation fee again. So at the very minimum you'll have to spend an additional $60 ($35 activation fee + $20 1GB data plan + taxes) in addition to the cost of the device.