Oddly, you do need the Blighty version of Android 1.5, but it will work okay in your US G1…provided you follow the darned instructions!

This is courtesy of bigfoot85t and Android Central, by the by, full disclosure and all that!

1. Head over to the T-mobile Forums to download the UK version of Android 1.5 Cupcake
2. Put the downloaded file into your SD card
3. Turn ur phone off and re-boot it holding the home button
4. When you see the triangle hit alt+L
5. Then alt+S
6. Press home and back when it tells you to (to reboot your phone)
7. It’ll say installing Radio after you hit home+back (thats normal)
8. It’ll reboot a couple times and after you see the new Android loading screen, you’ve got Cupcake.
9. When Android 1.5 for the US comes out repeat the process with the US download

Being the UK version of Cupcake, you see how hobo-fabulous us guys in the UK live, without snazzy things like Voice Search and Amazon MP3. Them’s the breaks, but you know that you can upgrade to the real deal when it does come out, so worry not and enjoy early!