Google says it received 4,601 requests for information on users from the US government in the last six months of 2010. That's more than double the number of requests from each of the next two countries, Brazil (1804) and India (1699). Google complied with 94% of the requests. The number does not reflect multiple requests for the same user so the actual number of individual requests could be much higher.
From the Googleblog:
Our goal is to provide our users access to information and to protect the privacy of our users. Whenever we receive a request, we first check to make sure it meets both the letter and spirit of the law before complying. When possible, we notify affected users about requests for user data that may affect them. And, if we believe a request is overly broad, we will seek to narrow it.