I found out what it was, at least on my vehicle: There is a design flaw in the fusebox located in the glovebox. Essentially what BMW did is make the fusebox connection to the battery cable a simple plastic clip, instead of a bolted down connection. When the car is driven, the battery cable gets jostled and electricity arcs between the connection, and does one of two things: fries the connection, or disconnects from the box. This can be temporarily fixed by opening the glove box, removing the fuse box cover, reaching around the back of the fusebox on the right hand side and jostling the cable until it regains the connection. This will at least make the car temporarily drivable, but it may lose its connection again.
When I got the car repaired, it was out of warranty, so it cost me a pretty penny. So be thankful yours is under warranty: it cost me $1200 to fix. It needed a new fusebox because my connection had melted, and labor to figure out the problem was the big cost because no one knows what the issue was.
But you'r right, it is a major safety concern, and I wish that I got reimbursed by BMW for what was an obvious design flaw. I'd make sure that you talk good and long with the service advisor at the BMW Service Center to make sure it won't continue to be an issue after your warranty expires. But this was a definitely one of the main factors in me getting rid of my '07 328i and trading up to an '09 335i.
Hope this helps.