I don't know what it is either... sometimes I get it right away and other times I end up yelling obscenities for about 10 minutes.
I've used a penny to open it (insert and twist), but it dings the plastic if you're not careful.
I had to exchange my Imagio, too. The cover on the first one was just getting worn down to the point where I could open it fairly easy, and now I have to start that process all over again...
I usually don't like to use ActiveSync, but I would rather deal with that than try to get that damned cover off.