This thread inspired me to pull a similar question off the back shelf, and check it out, but I haven't found the answer. Like the with you, System Properties shows only 3GBs of memory, out of 4GBs installed.
I ran the Windows Memory Diagnostic Tool, but it found no problems. I noticed that the initial BIOS screen showed dual channel, but only 3072MB okay, with 4096MB installed. This suggests that there is a memory problem, before the OS starts, but I also ran CPU-Z, and the current version shows 4096MB of memory. However, before getting the latest version of the program, I tried to run an older one, but Windows partially blocked it because it said that the driver wasn't signed. On this older version, it only showed 3072MB of memory, just like the BIOS screen.
Before installing the W7 RTM, this problem didn't exist, but I don't think that would have any effect of the BIOS readings. I'm going to run another test with Memtest86, but at this point, I'm not sure what is happening.