If you have a G1 and you are like me, then you have never gotten an OTA update from T-Mobile; my updates to RC28 and RC30 have always downloaded the file and performed a manual update. Good news; RC33 has been released (not quite Cupcake yet) and download available from:
The update includes some fixes tweaks, updates Google Maps to 3.0 (with Latitude) and voice search. More info here:
Basic manual update instructions:
Once the download finishes you'll have to save it to the root of your SD card and then rename it to update.zip. Reboot the phone holding Home+End once the recovery screen comes up, apply the update by pressing Alt+S.
More info on the update procedure: