Interesting. What rollback to 10.6.7 kexts did you try?
I have an XFX 5770 single-slot card, and an Apple Cinema Display 27-inch. Vanilla Lion gives me the black screen via mini DP + ACD-- but I can get video via DVI to a backup Dell monitor.
I swapped in the following 10.6.7 kexts (S/L/E) and now Lion boots fine, and my 5770 drives my ACD via mini DP:
ATI5000Control
ATIFramebuffer
ATISupport
After replacing the Lion kexts of the same name with those, I run multibeast just to get the cache rebuilt and call for repair permissions.
First boot, and all subsequent, are working!
There's another potential method... try to get in touch with the user Nihil on here. He's doing it with just 1 modified kext apparently. I haven't had a chance to give it a go.