The long awaited update for Android has been delayed a week so T-Mobile can finish the build. 
The rollout has been delayed by about a week, meaning all G1 owners should have it by early June instead of the end of May. Below is the announcement from T-Mobile.
G1 community,
We’d like to provide an update on the rollout of Android 1.5 for T-Mobile G1 customers.
We are working diligently to get Android 1.5 out as soon as possible, while aiming to ensure a consistent, positive experience for our customers. We’re finalizing 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.
Your device will notify you when the update is available. Thank you for your patience and understanding.
Now I really hope I am one of the first to get the update next week.