Had a similar issue. What I found consistently solved the problem (after much trial and error to validate) was to make sure that the default action settings for what happened when the usb is plugged in is either "charge only" or "disk drive". Then, if usb debugging isn't already on, turn it on. Easytether should work o.k. If it doesn't, uncheck "usb debugging" under the settings->application->development tab and re-check it. There was no separate "sync" option in 1.5/1.6 so I think that might having something to do with it. Probably easytether or driver related.