Sorry for the cross-post (http://stackoverflow.com/questions/8185773/eeepad-transformer-adb-problems-with-drivers-on-windows7-x64)... I was told this is a better place for my question.
I have 2 Windows7 x64 machines.
One of them has got both drivers (ASUS Android Composite ADB Interface and Transformer TF101) and when i run adb devices I get the device listed as expected.
The other one has only got one of the drivers (ie. the ASUS Android Composite ADB Interface is not there and there is no unknown devices or apparent problems in Device Manager).