It seems like common sense, but consistency really is the first rule of usability. Humans are creatures of habit. Once you've learned something, it's awfully difficult to change.
So why the, did Google decide to change their favicon? It boggles the mind. It mightn't be a problem for IE 6 users, but for Firefox users, the favicon appears on the browser's tab. Your site is identified by it's favicon.
It confuses the hell out of me (and it's not just me!). Now, instead of jumping back to my search results, I have to look around, go "where's that tab", then remember "Oh, Google's got a new fucking logo".
Google has effectively dealt a blow to your efficiency.
Demand the old favicon back.