If there is or there will be a beta key in existance, therefore there will be a PC client that plays the actual game outside of Blizzard computer systems. If that's the case, it will sooner or later make it to the torrents and be shared with most of people that are interested. Hacking things like that usually does not take longer than a week for someone who knows what they are doing.
Initially, I can see these keys selling for a couple hundred while quickly dropping in price. I'd guess most of them would sell between $20-50.
I think, the probability they will be giving those out at the upcoming BlizzCon is low. Most likely, if there will be a beta at all, it will be a couple of months before the official release, which is NOT in 2010.