I just got a dock for my Milestone and had problems with the touchscreen jumping all over the place when docked.
This is the dock I ordered:
Desktop Sync Cradle Dock For Motorola Milestone Droid on eBay (end time 25-Feb-10 23:52:36 GMT)
Although it actually looks nothing like the one pictured.
Anyway it turns out it only causes the problem when it is plugged in with the supplied UK mains cable (rated 5V 1A). Instead of using this I am using the supplied USB cable (different USB plug to the Milestone), plugged into the USB to UK Mains adapter which came with my Milestone (rated 5.1V 850mA), and now the touchscreen problems are gone!
Does anyone know if it might be the slightly different ampage/voltage of the supplied cable? Maybe I just have a defective unit, it was only 12 quid so I'm not really bothered.
Did some research on the forums with no luck, so I thought I'd post here in case someone else has the same problem