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After several failed attempts to install Boot Camp under Lion on my late-2009 27" iMac (black screen after a lot of the installation had apparently gone fine), I have finally succeeded. After the black screen episodes, I searched for help online and found this article: http://support.apple.com/kb/TS3173. That's where I learned that this problem is particular to 27" iMac, late 2009, and 21.5" iMac, Mid-2010. (Too bad you have to waste so much time before finding this out, but never mind.)
At first I found the article confusing because it refers to "steps A through N immediately below," which are instructions for formatting a USB storage devide as MS-DOS (FAT), and there are no steps A through N. Actually they're numbered 1-14. So I didn't need to do this because I already had a USB so formatted.
I followed the rest of the instructions exactly, and ta da!, it worked. How the installation manages to find the drivers on the USB, I have no idea. "It just works."
Please let me know if you succeed or if I can be of any further help. Good luck!