After quite a bit of screwing around I finally got my Galaxy S2 to be properly recognized under Windows 7 x64 for the purposes of using ADB. While this is primarily for software development, it's also important for flashing non-OTA firmwares, rooting etc.
If you have a Galaxy S3 I hear they have the same problem.
Note: This has nothing to do with mounting the SD cards in the phone as mass storage in My Computer so you can access them like a thumb drive.
If you have Debugging Options turned on in your phone and get a CDC_Serial and Android (or SAMSUNG_ANDROID) unknown devices when you plug in your phone in a x64 Win 7 machine then try this -
Download and install KIES from Samsung, this is the safest way to get the USB driver pack, rather than assuming all those links on the internets to strange locations are valid.
Run the USB Driver setup application located in program files (x86) samsung USB Driver. Install them where you want, they don't install per se, it's more an unzip process.
Plug your phone in. In Device Manager right click and Update Driver (after the fruitless search for drivers has failed).
Select Browse My Comptuer for driver software.
Select Let me Pick from a list of device drivers on my computer.
Click Have Disk
Select a file in the Escape_25 folder. You may have to try this step more than once, took me two different .inf files to get it to work.
You'll get back a list of Manufacturers (Samsung duh) and then the sub components on the right.
Pick the Samsung Mobile USB Composite Device in the right pane. You may to go through each Samsung entry in the left to find it.
Say yes when it warns that we're not sure this driver is for this device. Give it time to install, you can watch the process by the installation icon in the system tray. Just double click it to open it.
Repeat the process for the other unknown device. Let it install.
Now you shouldn't have any unknown devices in Device Manager and more importantly you should be able to do a -
and get the hardware ID of your device showing up.
Hope that saves someone else a few hours of WTF'isms.
Key words -
Device not listed when using ADB Devices
Unknown Device CDC Serial CDC_Serial Android SAMSUNG_Android
Samsung Windows 7 x64 USB Driver installation failure