Hello srq2625, and welcome to the Seven forums,
The file system in Windows 7 is not the much discussed WinFS, but the same old NTFS we all know and love
I think it may well be a later revision than the one shipped with XP but should not cause any problems, as it is fully backwards compatible.
The only issues I've seen with Drives formatted with XP, used on Windows 7, is the old one where permissions are still present on files from the original build.
This has cased users to find they cannot access certain files on the old XP formatted drive, from withing Windows 7.
This is easily solved by taking ownership of the file and adding the required security tokens for the Windows 7 users