I had the exact same problem. I'll step you through it...
Preperation:
Download DiskAid.
Download the MobileInstallation patch in the attachment (extract it somewhere you will remember).
Steps:
1. Your iPhone is in a constant reboot state, you only see the Apple logo-- plug your iPhone in to your computer.
2. Run DiskAid.
3. Change "Media Folder" in the bottom-left corner drop-down box to "Root Folder"
4. Read the dialog that pops up and click OK.
5. In the left-hand pane, click on System
6. Click on Libraries
7. Click on PrivateFrameworks
8. Click on MobileInstallation.framework
9. In the right-hand pane, click on "MobileInstallation", then on the top of the screen, select "Delete From Device".
10. Select "Copy File to Device" from the top of screen and navigate to where your downloaded "MobileInstallation" file is.
11. Click OK.
12. Leave your iPhone for a few moments, and everything should appear.
Video tutorial made by me here:
2009-01-21_1633