ok. My laptop is dell 630m/m140 with toshiba bluetooth stack installed. I followed the tutorial (
http://axim.dpicts.com/) to setup the Activesync between my X51 and the laptop. But the problem is I could not find the "ActiveSync" service on the PocketPC side, only the "serial port" service was available. I was so mad !!!
But FINALLY!!!

I have this issue solved:
1. right click the bluetooth icon on the notification area of your task bar
2. select "Service Properties"
3. enable PAN networking services!
Then it works!
I am so happy that I can use activesync through bluetooth now :)