I know the problem because i had the same problem.
I mailed to samsung Italy and UK and both told me there is no hardware combination of buttons to hard reset the device and i gave my device to the Assistance.
Then i retired the 780 three days ago and now it's working.
But this is strange because every Pocket PC has got buttons combination and i asked to the Assistance in which way they resolved my problem and they just told me "software upgrade..."
I thought, if they can why we can't hard reset the I780 ?
At the end i'm sure there is a buttons combination or a little "trick"... on the contrary Samsung Assistance changed with my device with another one.
I asked for this combination in my italian forum and in others international forum but, at the moment, it remains a mistery....