For the past week my X51v has refused to connect to my PC. I tried everything, soft/hard resets, even an online chat with a Dell technician, who advised me to reinstall XP to my desktop. I was convinced, however, that the problem was with the X51v and I was not going to format 200GB of hard drive and re-install my original Windows XP OS. I almost got to the stage where I was going to send the Axim back to Dell and demand a replacement.
As a last resort, this morning I went through the 'Sticky' threads and followed a link to Microsoft's Active Sync Help Centre. Although I had not been anywhere near my PC firewall settings for months (actually, ever) I followed the advice and checked the exceptions. Sure enough, the 'don't allow exceptions' checkbox was ticked. Unchecking it produced the reassuring chirrup from PC and Axim indicating the were talking to each other again.
I have no idea how the checkbox became checked; I certainly didn't do it. But the moral of this story is, don't assume just because you have not done something that your computer has not done it on its own!