Did you try a restore?
Simply activate sbsettings, power, safe mode. Once in safe mode, "restart" and you'll be up and running again - no quarter screen issue.
This is caused by an issue with activator when the phone is rebooted.
This does fix this issue, and is described in many blogs. However it fixes it only temporarily, and has to be repeated any time you reboot.
What baffles me is the cause of this issue. reptrich, author of Activator, has claimed this is NOT related to Activator. So what is it??
I have just got this same issue - only in the phone App - on a non-jail broken iPhone 4.