Hey, I'm having the same issue but the available for me is different. I have a Toshiba laptop A305D-S6848 which originally came with 3 GB of Ram. Since then i've added another 1GB for a total of 4.
Up until this past weekend, I was running Vista x64 and it reported 4GB ram. This weekend however, I installed Windows 7 x64 and it reports 4GB Ram but only 3.87 GB usable.
I also have a desktop machine with 8GB Ram and it didn't specify whether or not some or all of the Ram was being used so i assumed that 100% is usable.
NOW, another thing i noticed is whenever i do a fresh install, i always give the OS the satisfaction of creating it's own partition, doing this in Windows 7 however, it also created a smaller 100MB partition for system resources. I'm under the assumption that if you don't allow Windows 7 to create this seperate partition, it will seperate a section of Ram for this. If this is the case however, then both the partition and the Ram have been seperated for the system. Again, this is just a theory, I could be completely wrong...
AJ