Treo 680 is an ACTIVE USB device, that is, it makes contact to the host computer even before you press the SYNC button.
This can cause strange results if you connect through a USB hub, for example. I have a 680 - used to have a 650 - and am finding similar 'problems'. My 650 (and I think 700p would be the same) was eaiser since it was a PASSIVE device. And I just had to press SYNC, select it in the DEVICE list, cancel Sync (unless I could do the first two steps in less than 7 seconds or so) then press SYNC again. The second press is necessary because the passive Palms, after their initial contact attempt, go into various alternative modes that are not likely to ever connect.
Anyway, for the passive devices, having the USB set to autoconnect should work best. It should work fine for the active devices too.
The down-side is every OTHER USB device you plug in will first go to Windows. And that could really confuse cameras and some USB keys.