As I mentioned in this answer, I applied for access on the 13th of December, and then received an email saying I'd been approved on the 17th of December.
I have an app on the app store which used to use Google Maps, but I had to remove the maps when iOS 6 came out. Not sure if this had anything to do with it, or if I was just lucky because I happened to hear about the SDK almost immediately after Google announced the new SDK, and I applied for access right away.
When I received the email saying that I'd been approved, I was added to a google group - which is how Google sends out notifications of updates. From the looks of it that group only has 77 members, so if that's equal to the number of people which have been given access, I guess Google are rolling out access quite slowly.
UPDATE:
As of the 21st of February, it looks like the SDK is now open to everyone.