I tried on my test system - IBM ThinkCentre (8183-32u) - Pent 4 - SSE2 - 1GB ram
Video card is Diamond Stealth Radeom 9250 - 256Mb ram - PCI
To be sure I had no residuals from Callisto, I booted under my maintenance partition and removed all files from /System/Library/Extensions, restored the group in my ExtensionsBackup folder and then added XRadeonFB.kext. I removed the Extensions.kextcache & mkext - set ownseship and permissions.
When I boot from my normal boot partition the system shows the gray Apple with pinwheel. Then there is a flash to black for about a second followed by the gray Apple & pinwheel again. Then all disk activity stops.
When I try to boot the system with the -v flag I can see some messages about finding the ATI 9200 Pro, and a huge flood of r2d2 messages. Then it stops after a message about launching the login window. There are several (7 or 8) messages similar to:
(89) .. SystemStart ... fail (0) tfp
Sorry I didn't write it down exactly - I should have

If I boot the maintenance partition and remove the XRadeonFB.kext file from Extensions folder the system boots correctly.
I see from the info in the System Profiler - > Software Extensions that some of the extensions say "No valid version of this dependency can be found". However a version of the extension is loaded. It looks like things might work if all the versions fit togather.
Any comments on how to resolve the miss-matched versions?
System specifics:
CPU Type: IntelŪ PentiumŪ 4 CPU
Number Of CPUs: 1
CPU Speed: 3.4 GHz
L2 Cache (per CPU): 512 KB
CPU Features: FPU VME DE PSE TSC MSR PAE MCE CX8 APIC SEP MTRR PGE MCA CMOV PAT PSE36 CLFSH DS ACPI MMX FXSR SSE SSE2 SS HTT TM EST
Memory: 1 GB
Bus Speed: 533 MHz
Boot ROM Version: 2AKT42AUS (IBM)
Serial Number: 6A57448
Display:
Type: VGA-Compatible Controller
Bus: PCI
Slot: PCI Slot #2 - J901I
VRAM (Total): 128 MB
Vendor: ATI (0x1002)
Device ID: 0x5960
Revision ID: 0x0001
Displays:
Display:
Resolution: 1280 x 1024
Depth: 32-bit Color
Core Image: Not Supported
Main Display: Yes
Mirror: Off
Online: Yes
Quartz Extreme: Not Supported
clip from System Profiler - > Extensions
ATIRadeon8500:
Version: 1.4.26
Last Modified: 10/7/06 1:40 PM
Get Info String: ATIRadeon8500 1.4.26.11 (3493)
Location: /System/Library/Extensions/ATIRadeon8500.kext
kext Version: 4.2.6
Load Address: 0x2f621000
Valid: Yes
Authentic: Yes
Dependencies: Incomplete
Dependency Errors:
com.apple.iokit.IOGraphicsFamily: No valid version of this dependency can be found
com.apple.iokit.IONDRVSupport: No valid version of this dependency can be found
com.apple.iokit.IOPCIFamily: No valid version of this dependency can be found
Integrity: Kext is modified
IOKit:
Version: 8.4.1
Last Modified: 10/7/06 1:40 PM
Get Info String: I/O Kit Pseudoextension, Apple Computer Inc, 8.4.1
Location: /System/Library/Extensions/System.kext/PlugIns/IOKit.kext
kext Version: 8.4.1
Load Address: 0x0
Valid: Yes
Authentic: Yes
Dependencies: Satisfied
Integrity: Correct
IOKit6.0:
Version: 7.9.9
Last Modified: 10/7/06 1:40 PM
Get Info String: I/O Kit Pseudoextension, Apple Computer Inc, 7.9.9
Location: /System/Library/Extensions/System.kext/PlugIns/IOKit6.0.kext
kext Version: 7.9.9
Load Address: 0x0
Valid: Yes
Authentic: Yes
Dependencies: Satisfied
Integrity: Correct
IONDRVSupport:
Version: 1.4.3
Last Modified: 10/7/06 1:40 PM
Get Info String: 1.4.3, Copyright Apple Computer, Inc. 2000-2004
Location: /System/Library/Extensions/IONDRVSupport.kext
kext Version: 1.4.3
Load Address: 0x2f5a0000
Valid: Yes
Authentic: Yes
Dependencies: Incomplete
Dependency Errors:
com.apple.iokit.IOGraphicsFamily: No valid version of this dependency can be found
com.apple.iokit.IOPCIFamily: No valid version of this dependency can be found
Integrity: Kext is modified
IOGraphicsFamily:
Version: 1.4.3
Last Modified: 10/7/06 1:40 PM
Get Info String: 1.4.3, Copyright Apple Computer, Inc. 2000-2004
Location: /System/Library/Extensions/IOGraphicsFamily.kext
kext Version: 1.4.3
Load Address: 0x2f568000
Valid: Yes
Authentic: Yes
Dependencies: Incomplete
Dependency Errors:
com.apple.iokit.IOPCIFamily: No valid version of this dependency can be found
Integrity: Kext is modified
IOPCIFamily:
Version: 2.0
Last Modified: 10/7/06 1:40 PM
Get Info String: 2.0, Copyright Apple Computer, Inc. 2000-2004
Location: /System/Library/Extensions/IOPCIFamily.kext
kext Version: 2.0
Load Address: 0x2486f000
Valid: Yes
Authentic: Yes
Dependencies: Satisfied
Integrity: Kext is modified