I've been doing some testing lately (and still am) and I noticed a couple of things.

I have a FX5500 128MB AGP card installed on this machine, and it's supported with NVinject, NVKush and NVdarwin on 10.5.5 and lower.

However, upon updating to 10.5.6, I noticed that I could not boot, and it was always after those injectors did their probing. When I removed them, I could then boot, without acceleration.

This was tested on another HD, so I booted into my 10.5.5 system, and copied the old nvidia kexts I installed when I updated to 10.5.5 and moved them into the 10.5.6 system and it booted.

Now, I noticed that many people with 6XXX cards or higher can boot fine after updating. So this must be a <6XXX card issue.

If you have an FX5200 card or higher, but not a 6XXX card, I suggest trying the kexts from 10.5.5 if you can't boot after updating to 10.5.6. Make sure it's the injector at fault by booting with -v and watch when the injector does it's magic. It should hang shortly after.

Please let me know if this fixes your boot problems.

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Mac OS X Leopard 10.5.8 (9L30) | Chameleon 2.0 RC3 | Intel Pentium 4 3Ghz 800Mhz FSB HyperThread 1MB L2 cache SSE3 1MB L2 cache works, cosmetic display of 512KB L2 cache | Asus P4V8X-MX VIA Chipset, VIA-VT8237 Southbridge | AC97 VIA8237 | Dual 1GB 333Mhz DDR SDRAM | nVidia PNY GeForce 7600GS 512MB 8x AGP DVI/TV-Out/VGA [NVinject 0.2.1; QE/CI/QuartzGL/Rotation] | Darwin Kernel Version 9.7.0: Sun Jun 14 20:48:28 IST 2009; Voodoo 2.0 Intel alpha3 :xnu-1228.12.14/BUILD/obj/RELEASE_I386 i386

Got the same problem with my FX5200, panic with nvkush, gonna give it a try with 10.5.5 kext.