I signed up with linode when they did a similar promotion about a year and a half ago, and it clearly said that I was in before the promotion ended and would be getting the bonus before I actually paid for it. I assume it will be the same way this time.

From the comments:

"The black ‘giveaway’ box on the signup page carries through the entire signup process – and will refresh to show how many Giveaway Bucks are remaining – so you’ll be able to tell BEFORE you complete the signup how close to the wire you are. If you don’t make the cut and don’t want the service and cancel we’ll refund your money. We’re not out to screw anyone."

I'm just hesitant after reading the comments here.

It's a brilliant promotion for them. Cheaper than AdWords and guarantees either new customers or upselling existing customers.