T-Mobile is currently sending out an OTA update to select G1 users, but it isn't the much-anticipated Cupcake release. It should have some pleasant surprises though—not least of which is Google Voice Search. Updated
T-Mo news also claims that the firmware, version RC33, will allow long-click saving for images and files, as well as manual update checks. Naturally there will be quite a few bugfixes as well, but T-Mobile hasn't published full release notes yet.
An administrator at the official T-Mobile forums has confirmed the release, but gives a slightly different rollout schedule than our tipster, an employee at the company. The tipster laid out a February 5th-15th rollout, whereas the forum admin claims that updates are seeding to users right now, and will complete in about two weeks. Either way, Google Voice Search for the iPhone is fantastic, so this is a nice treat for Android users. [T-Mo Official Forums, T-Mo News —Thanks, DJ!]
Update: Confirmed, the over-the-air update will be rolling out to customers this week in a staggered release, and will continue over the span of the next two weeks.