[G1 General] brandenk:
Walkthrough: Turning a T-mobile G1 into Dev phone on AT&T

Listen to me at your own risk, this is what worked for me.

Ok, so I bought a T-mobile G1 off of craigslist for $120.

Then I went to http://www.unlock-tmobileg1.com/procedures/activate.php and followed it step by step. I needed to borrow a T-mobile SIM card from a buddy at work and once I got the code and paid $24 USD I had the unlock code and my phone unlocked in about 15 total minutes!

Menu button > Settings > Wireless Controls > Mobile Networks > Access Point Names > Menu Button > New APN

Name: AT&T

APN: wap.cingular

Proxy: leave blank

Port: leave blank

Username:WAP@CINGULARGPRS.COM

Password:CINGULAR1

Server: leave blank

MMSC: http://mmsc.cingular.com

MMS Proxy: wireless.cingular.com

MMS Port: 80

MCC:310

MNC:410

APN Type: leave blank

when done "Menu Button" then "Save APN"

Next step was flashing from the OTA RC30 back to RC29, you can find out what version you have by going to Settings > About phone > Build number, it will say alot of things but in there somewhere it will either say RC30, RC29, RC8, or devphone. The goal is to get to RC29 so you can get root access. First I followed this walkthrough: http://android-dls.com/wiki/index.php?title=Root_For_RC30 Cliff notes: put a file on your SDcard, reboot your phone holding camera and power, then follow the instructions to upgrade.

After you have RC29 you need to keep root, you can do this by following this walkthrough: http://android-dls.com/wiki/index.php?title=Rooting_Android but stop at the Add Busybox section, you dont need that. Cliff notes: reboot your phone, open the keyboard and press Enter Enter telnetd Enter then close your phone, go to the market to get the telnet app.

Now plug your phone into your computer and mount through USB. Go to this link: http://android-dls.com/wiki/index.php?title=Keeping_Root and follow the walkthrough to get JKv1.3 installed on your phone. Download AndroidMod1.0.zip and you only need the file named recovery_testkeys.img, copy it to your SDcard (which should already be in FAT32 format from the first step). Then download JesusFreke's JFv1.3, pick your version by clicking on the name you want, RC30, RC8, ADP1, I chose the ADP1 version because its the development version and its more open and you dont get amazon and IM, it also will say in your Model number Android Dev Phone 1 and under Build number it will say devphone. After you download that file name it update.zip and put it on your SDcard, so now there are 2 files on it. Then open that telnet app, connect to local host and type those root commands in there from the walkthrough. Then follow the final steps with booting holding home and power, when you get the image press ALT + L together, make sure it says test-keys on the second line, dont worry about the top line. If it does say that press ALT + S and sit back and relax for a bit until it tells you to press Home and Back. Once you do that it will reboot twice and you will eventually make it back to the initial setup screen.

And the final step after you phone is setup is to install the Engineering Bootloader. Follow this link: http://xdaforums.com/showthread.php?t=455860, download the new Hard SPL version, connect your phone via USB, copy it to the SDcard, and make sure nothing else is there, then rename it update.zip. After you do that turn your phone off, then hold house and turn it on to get to the recovery screen, press ALT + S and wait til you are told to press Home and Back and you are done. If you want to verify turn off your phone and turn it on agian holding camera and power. Confirm that the second line reads HSPL10.95.3000

If you do all these steps your phone will be identical to mine. :D