Sorry, folks. T-Mo has announced a delay in the OTA Cupcake update. See the link for the announcement over at T-Mobile forums. Man, oh, man. Early June. Stop playing with our emotions!

"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."