Files are redirected to VirtualStore on Vista and 7, but
only when UAC is enabled (which is the default, and strongly recommended); if UAC is disabled or the program is running with admin elevation, files are stored in the real location. VirtualStore is preferable, however, as it means you can back up all your saves and settings simply by copying your user profile. On XP and earlier, backing up saves and settings was incredibly frustrating and time-consuming as every game had its own way of organising them.
PieceMaker42: Occassionally a game will save its files to C:\Users\<username>\Saved Games ... but no GOG have saved here for me.
GOG does not change the internal behaviour of the games being sold. Only newer games make use of the saves directory; this standard is encouraged by Microsoft and is a requirement of the Games for Windows brand, so in the future only non-compliant games will store files elsewhere.