As you already know I'm a big of panning shots as well.
The shutterspeed depends on how fast your object moves, the faster they move and the lower you go on the shutterspeed the blurrier your pics will be. Or you have a very very steady hand
I have taken panningshots from 1/60 to up to 1/320 but I prefer 1/200. Here are a few examples.
M3GTR: Taken @ 1/200
Lotus Elise 1: Taken @ 1/80
Lotus Elise 2: Taken @ 1/80
Viper Zakspeed Racing: Taken @ 1/200 High Res
http://users.pandora.be/jokke1/Photo...66%20Viper.jpg
VW Bora TDI: Taken @ 1/160 High Res
http://users.pandora.be/jokke1/Photo...20TDI%20lr.jpg
Enjoy the pics
Cheers