Flash vulnerability on Vista. 
Yep, but if you read this:
http://blogs.zdnet.com/security/?p=993&tag=nl.e539It seems that the flash vulnerability could have taken down any of the three.
"The flaw is in something else, but the inherent nature of Java allowed us to get around the protections that Microsoft had in place,” he (Macaulay) said in an interview shortly after he claimed his prize Friday. “This could affect Linux or Mac OS X.”
Macaulay said he chose to work on Vista because he had done contract work for Microsoft in the past and was more familiar with its products.
Aha, so there is your story right there, this flaw could’ve worked on any of the systems; however, the contest rules state that the same exploit can only be used to compromise one machine (see rule #2 from the cansecwest.com web page which states “You can’t use the same vulnerability to claim more than one box, if it is a cross-platform issue.”), and Macaulay used Vista because it was what he was more familiar with."
Update, Sorry for posting this, I missed Franks post on the same thing.