Registrations are now open for Google I/O 2016, and Google isn't taking them on a first come, first serve basis. You'll be able to submit a registration application from March 8 through March 10 at 5 p.m PST, and then Google will randomly select winners. In the past, Google just let people purchase tickets until they were sold out, but it was less than ideal for everyone involved. From Google:
The window to submit a registration application will open at 9AM PST on March 8, 2016 and remain open until 5PM PST on March 10, 2016, so you'll have plenty of time to apply. We'll randomly select applicants from among all of the qualified applications once the window closes on March 10, 2016 and notify those selected via email. The order in which registration applications are received has no bearing on the final outcome.
Keep in mind, if selected you will need to fork over the $900 ticket fee. Don't worry if you can't attend in person though, as Google will again be livestreaming various portions of the event on its site.