I work at Google and was the one who posted on our forums about this.
What our systems found was definitely a compromised JS file, and others on this thread have posted something similar to what we saw. This is not a false positive.
We have detailed help for webmasters in this kind of situation:
One thing that I strongly suggest to any webmaster in this situation is to look for any server vulnerability that allowed this file to get compromised in the first place. We sometimes see webmasters simply fix the affected files without digging into security hole that allowed the hack, which leaves the server vulnerable for repeat attacks.
What our systems found was definitely a compromised JS file, and others on this thread have posted something similar to what we saw. This is not a false positive.
We have detailed help for webmasters in this kind of situation:
http://www.google.com/webmasters/hacked/
One thing that I strongly suggest to any webmaster in this situation is to look for any server vulnerability that allowed this file to get compromised in the first place. We sometimes see webmasters simply fix the affected files without digging into security hole that allowed the hack, which leaves the server vulnerable for repeat attacks.
Happy to answer questions.