I hear lots of whining about Steam support, to almost legendary degrees. This was my first experience dealing with them and they were amazingly quick, and they painlessly fixed the issue.
I've heard some good stories about them, too -- glad to add yours to the list.
Just wish they'd perma-ban the *expletive deleted so I don't get in trouble* who nicked it.
That's always a tricky business. While I agree with you, Steam is in the unfortunate position of not being able to verify for themselves that the other person was actually a thief. For all they know, Arcen accidentally gave out two copies of the same key to two different customers, or some other vendor did, and thus they'd be banning another legitimate customer who would then be even more irate with them than you are with the presumed-thief.
So in their cases, they err on the side of caution rather than being too quick to ban, which I think is good. Without asking the other party for proof of purchase of some kind (which the other party might legitimately not be able to provide even if they were not a thief), there's not any way for Steam to directly figure out if they are a thief or not. What they did do is figure out that you are
not a thief, which is the important part from their end, I guess.
For what it's worth, I did check with Impulse to make sure there were no duplicate keys in their list or duplicates given out in general, and they said that there were not. I trust them on that. With GamersGate and some of the others, I also don't have any reason to believe there were duplicates -- and I'm quite regimented on how I keep track of which key banks go to which vendor, etc. So, personally, I am pretty darn sure that other person actually is a thief, but Steam doesn't have the direct data to share in my confidence, and they have more to lose if they perma-ban the other player and it turned out to be a misunderstanding of some kind (though Arcen would probably lose a customer out of that, too).
So... all in all, I think Steam is doing a good job of making the best out of a bad situation, since they got you set up.