What makes a bet valid?

Yesterday on my coffee break I made a bet with one of my co-workers regarding the validity of a rumor I heard about Brett Favre's retirement.

Here's how it went down:
me: I heard Brett Favre is retiring
co-worker: No way. I'll bet you.
me: how much
co-worker: ten bucks
me: done
co-worker: wait no make it 5.

So now that there have been more semi-official reports come out regarding the retirement my co-worker is trying to get out of said bet using the following loop-holes.

• We didn't shake on it. I call verbal contract. I have witnesses. So what if we didn't shake.
• It's not really a retirement, if he comes back. I'm willing to allow that unless he actually sits out the first game of the regular season it's not really a retirement. However in the world of sports when a comeback is made it's called coming out of retirement, thus one would have to have been retired to come out of retirement.

So now I pose the question do I have a valid claim on $10, assuming of course that Favre does in fact sit out of the first game of the regular season.