Hurrah, this is a problem I have come across on my G1 a few times usually after the phone has powered off due the battery running out and each time I have had to hard reset to fix 🙁 but today I found a way to restore functionality without having to reinstall everything.
Go to the market and install “Anycut” (which is free) then open it and create a new shortcut to an action. In the action list scroll down to Setup Wizard (there will be about 4 of these) and select the last one. Accept the name at the prompt and then exit to the home screen by using the back button repeatedly.
Anycut is no longer in the market but there is another free application called “dg QuickCut” which will do the job.
Install “dg QuickCut” and run it, you will then want to “create a desktop shortcut”
Scroll down to Setup Wizard (there are three of these on mine) and select the last one.
Return to the home screen and select the Icon you have just created.
This will bring up the setup wizard, go through it and sign into your Google Account
After doing this it will start looping, ie going back to the sign in but this does not matter as you can press the home key AND IT WILL WORK!
If the short cut takes you a point half way through the setup wizard, run through it and then go back into dg Quick Cut and select a different startup wizard entry.
Find the Setup Wizard icon on your homescreen (it will be where ever it fits) and run it, you may need to sign into your google account again and just follow the steps, when it exits your phone should be working again 🙂