This is a short howto for installing an Android 2.1 rom (e.g. OpenEclair) on the HTC G1/ADP1/Dream.

First of all you the Amon Ra recovery tool.
A howto on installing this can be found here

Notice: To be able to continue this howto you need at least amon ra 1.3.2.
I used the recovery-RA-dream-v1.5.2.img. You can find more information on the xda-forum

Then you need to format your SD card. ( Backup your SD card to your computer as you will format your SD! )
A video howto can be found here

But here you have the written one:

• Boot in to recovery mode ( HOME + END )
• Select "Partition SDcard"
• Select "Partition SD"
• It will ask you to confirm by pressing HOME. ( So hit HOME )
• Then you will be able to select the swap-size. Use the trackball to change.
  Select the preferred value ( I selected 128MB ) and hit HOME.
• Then you will be able to select the ext2-size. Use the trackball to change.
  Select the preferred value ( I selected 640MB ) and hit HOME.
• It will ask you to confirm by pressing HOME. ( So hit HOME )
  Now the formatting starts and will show you some progress - dots.
• Select "SD: ext2 to ext3"
• It will ask you to confirm by pressing HOME. ( So hit HOME )
  Now the convertion starts and will show you some progress - dots.
• Select "SD: ext3 to ext4"
• It will ask you to confirm by pressing HOME. ( So hit HOME )
  Now the convertion starts and will show you some progress - dots.
• Hit BACK
• Select "Go to console"
• Hit Enter
• Type "reboot recovery" and hit Enter
  This will reboot your device directly in recovery mode.

Now you can start the installation of your new Android 2.1 rom. Here I will use the OpenEclair 1.2.1.

A video howto can be found here

On your PC

Get all files you need:

• Latest radio image: ota-radio-2_22_19_26I.zip
• Danger/Death SPL: spl-signed.zip
• Android 2.1 based rom: OpenEclair-v1.2.1.zip

Connect your phone to your computer.

On your phone

• Select "USB-MS toggle"
  This will mount your SD card to your computer.

On your PC

• Put all 3 files you just downloaded on the root of your SD card.
• Unmount/eject the SD card.

On your phone

• Hit HOME.
• Select "Wipe"
• Select "Wipe data/factory reset"
• It will ask you to confirm by pressing HOME. ( So hit HOME )
• Hit BACK
• Select "Flash zip from sdcard"
• Select "ota-radio-2_22_19_26I.zip"
• It will ask you to confirm by pressing HOME. ( So hit HOME )
• It will ask you to reboot by pressing HOME + BACK. ( So hit HOME + BACK )
  This will reboot your phone and will bring you back to the recovery mode.
• Select "Wipe"
• Select "Wipe data/factory reset"
• It will ask you to confirm by pressing HOME. ( So hit HOME )
• Hit BACK
• Select "Flash zip from sdcard"
• Select "spl-signed.zip"
• It will ask you to confirm by pressing HOME. ( So hit HOME )
• It will ask you to reboot by pressing HOME + BACK. ( So hit HOME + BACK )
  This will reboot your phone and will bring you back to the recovery mode.
• Select "Wipe"
• Select "Wipe data/factory reset"
• It will ask you to confirm by pressing HOME. ( So hit HOME )
• Hit BACK
• Select "Flash zip from sdcard"
• Select "OpenEclair-v1.2.1.zip"
• It will ask you to confirm by pressing HOME. ( So hit HOME )
• It will ask you to reboot by pressing HOME + BACK. ( So hit HOME + BACK )
  This will reboot your phone and will bring you back to the recovery mode.
• Tab the android.
• Select "Skip"
• Select "Skip"
• Select "Next"

You're now at the home screen of your new image. If want to setup your google account, do this by the marker.

P.S: Try this at own risk. It is possible that you brick your phone!