True, this is possible, and they could combine this information with IP + UA when you are visiting google properties that also have the google analytics code in it.

Though depending on the network you're on (how many people are NAT'd behind that IP), and how generic your User-Agent string is, this could be useless. Comparison from Urchin (owned by google): http://www.urchintools.com/urchin6/configuration/tracking-me...