We have seen this behaviour on one client’s WM6.1 device and did not think much of it other than it happened twice in the last three months.
Today, the settings reset to Manual Updates only on my own personal WM6.1 device too. There are no behaviours associated with the reset other than perhaps the phone needing to be reset after it had locked up.
So, if a client calls stating their phone seemingly stopped receiving e-mail, the first thing to check before resetting it is the synchronization schedule and whether it has been reset recently.
It very well may have.
Microsoft Small Business Specialists
Co-Author: SBS 2008 Blueprint Book