Follow the first section "Preparing to Get Root on your Phone" at this page, http://forum.xda-developers.com/showthread.php?t=442480.
For some reasons, it took me many repeats for this to be successful. I tried Linux downloading, windows downloading, formating FAT32, etc. In the last shot, it worked, I still don't know which factor made the difference. In any case, before the last successful downgrade, I certainly did a complete FAT32 reformating, the image was the only file on the disk and the downloading to SD card was done on windows.
The next section talks about using an app "telnet" to gain root access. But that requires a lot of typing on the phone. Plus, if you got a brand new phone from internet, you may have not registered it yet.
So I use adb+busybox as follows:
You can test to see if the blind typing is working or not by typing and see if phone reboots. If it does, then you can do above step with confidence.<enter><enter>setprop persist.service.adb.enable 1<enter>
adb shell mkdir /data/local/busybox adb push busybox /data/local/busybox
Now you should be root as you can see "#" prompt sign!cd /data/local/busybox chmod 755 busybox ./busybox telnet 127.0.0.1
Follow the second section "Rooting your RC29 or lower phone" at this page, http://forum.xda-developers.com/showthread.php?t=442480, except that you do all the commandline commands from rooted telnet window from adb shell instead of "telnet" app as stated in the link.
Next you want to replace the bootloader so that you can flash your own kernel and do more adanvced stuff. Follow the third section "What To Do Now That You Have Root" at this page, http://forum.xda-developers.com/showthread.php?t=442480.
To ensure you did it correctly, turn on the phone while pressing camera button, and you should see three androids standing on skateboards, a different picture from the previous image.
Simply go to http://android-dls.com/forum/index.php?f=24&t=241&rb_v=viewtopic and pick the right zip file, put on your sd card, rename to update.zip, and use the recovery image to update the userland. Make sure do a wipe (Alt+W) before updating (Alt+S).
I picked JF v1.43 Holiday release for ADP1
Follow the instructions at http://forum.xda-developers.com/showthread.php?t=452316
If you just pop in any SIM card, then you don't need to deal with the first step, getting around "no sim card found" problem, which will save you a lot of effort. Just make sure clicking on "dismiss" when prompted to unlock SIM.
The only thing you really need to do is to enable wifi. Type the following from the adb shell:
Then you will the setup app running on your phone, and you can turn on wifi there.am start -a android.intent.action.MAIN -n com.android.settings/.Settings
After that, you can proceed to register with Android market.
If you are here, congradulations! You have just converted a regular G1 phone to ADP1 phone. Now you can root, change kernel, or change just about anything you want while you are still able to install all Android apps. Not sure if you can buy paid Android apps, tho. Please let me know.