Are you using the 32bit or 64bit version of Vista?
I belive there is a limitation in the 32bit version which limits you to using 3GB of RAM (even though theoretically you should be able to access 4GB).
edit: found a Microsoft knowledgebase article about it: http://support.microsoft.com/kb/929605
The reduction in available system memory depends on the devices that are installed in the computer. However, to avoid potential driver compatibility issues, the 32-bit versions of Windows Vista limit the total available memory to 3.12 GB. See the "More information" section for information about potential driver compatibility issues.
They also list a workaround which involves going into the BIOS and playing around with some settings. However, trying to do that on a iMac that uses EFI instead of BIOS might be a bad idea... I don't know exactly how Boot Camp fools Windows into thinking there is a BIOS, or if there is some kind of legacy BIOS emulation in EFI. Either way, it's not something you want to mess with unless you don't mind breaking your new iMac