One of the best ideas I've ever heard about-- there's serious potential there.
I'm married, and can attest to a difference.
I'm not sure if it's the availability that's the issue. I think there are two things working here. One is desperation, or rather, the lack of it. Single people may come across as needier, which is a turn-off to just about everyone. I've found that the best times for opportunities have come in the weeks following a break-up of a major relationship where I'm available, but not actively looking.
The second is having references. Has anyone else found that picking up is so much easier at a house party than at a bar, even if the atmosphere is similar? I think that that's because at a house party, you've got to at least know someone who knows the other person, so you can be referenced, found again, even if that connection is often abstract. Having a significant other or a female friend is just as good for the same reasons. Somebody else thinks you're good material and not some crazy person off the street.
MC is right, says the married woman who gets hit on all the time. It's all about just not caring as much. It certainly isn't some "chicks dig unavailable guys" misogynist crap. People find desperation unattractive, and the confidence that comes from not being desperate, irresistable--at least if the non-desperate, i.e., coupled person, is willing to flirt.
So why do chicks dig unavailable guys?
Seriously, we should also credit the "stamp of approval" reason: if he's with someone, odds are much better that he's not an ax murderer (of which there are fewer and fewer nowadays...I blame the NRA).