I had a similar problem with an upgrade, and as I recall, the solution was to manually switch back to an earlier kernel. If you've got your original kernel handy, you could try that; or you could try one or more third-party kernels until you find one that works. You can specify the kernel as part of the boot loader options (I don't recall the exact syntax), or you can use an emergency boot (say, of your installer) and copy the new kernel over the old one. (I recommend backing up the original first, of course.) The default kernel name is mach_kernel, and it's usually located in the root directory of the main OS X filesystem.