Google I/O is a very popular event with a very limited number of tickets available. If you want to attend this year set your alarm for REAL EARLY this Monday to jump on your computer and grab tickets the minute they go on sale. The AndroidDev team just tweeted tickets will go on sale this Monday. Their tweet adds, "They won't last long."
Past attendees of Google I/O have already had their chance to register, and on Monday it will be open to the general public.
This years event will be held at the Moscone Center in San Francisco on May 10th & 11th. Check out the Google I/O webpage for all the details.