It seems that T-Mobile has recently announced that they are delaying the release of the Android 1.5 from May to June.

However, G1 owners need not be too discouraged as the updated was delayed just a week with a T-Mobile forum moderator by the name of Dominic giving the following statement:

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.

So, mark your calenders, Android 1.5 will officially be released for the T-Mobile G1 in early June.