Android Community is hearing from multiple T-Mobile G1 users that Android 1.5 "Cupcake" is being pushed out as an OTA upgrade. The system update, which was released officially to developers earlier this week, will be rolled out to handsets throughout May, according to one report.
Reports are telling us that the update is reaching users in the US and Europe. The Android 1.5 system update is a 44MB file, and as yet there's no way to artificially trigger its download, only wait for your carrier to push it OTA. Keep track of progress in this forum thread.