G1 owners: Some people got it already, but expect the Cupcake general public rollout to officially start next Tuesday. According to one reliable source, that will be C-Day. This goes along with T-Mobile's previous statement about the delivery of Android's new version:
We're finalising the build this week to ensure optimal functionality and smooth delivery. Therefore, the rollout schedule has been reset by approximately a week, and we expect all G1 customers will have the update by early June.
So yes, if it starts rolling out next week, they will meet their "all G1 customers" by early June target.