Update: The Wall Street Journal is now reporting this as well, saying “Google Inc. is in talks to acquire popular video-sharing site YouTube Inc. for roughly $1.6 billion, according to a person familiar with the matter. The discussions are still at a sensitive stage and could well break off, this person says.” More here.
We know that YouTube has had informal talks with a number of companies about acquisition in the $1.5 – $2 billion range. And I suspect Google won’t be daunted by the prospect of dealing with a ton of pissed off copyright holders.
Based on experience with these sort of rumors, I’d put this at 40% likely to be at least partially true.