curiously did you also get a white screen during the installation?
Several have poste on the 68xx (particularly 6870) with this issue -
Me too. Sometimes Multibeast 4.6.1 would fail during the registration step, and then I would hang endlessly on the loading icon.
I ended up yanking out all my HDD and SATA3 SSD (with 10.7.4) on it.
If it spins endlessly, I have no choice but to reinstall from my USB stick...
after the apple logo appeared, i got the white screen and cursor on my radeon 6870
i disabled the discrete graphics card in bios and used the built in graphics on the mobo during install
others removed the discrete card during install of ML