I've noticed this happening a few times since ICS. Its happened a few times during navigation and its almost like the map freezes and I've also noticed when not in nave mode, the gps icon in the upper left corner of the status bar blinking and "searching for GPS". As a side note, it seems like when this happens, it drains my battery much faster. It seems like this happens under two circumstances. 1. If I'm in a building with poor signal (GPS and CELL) and 2. after using the facebook app (which I now "kill" so it doesn't auto launch).
My "fix" is quite simple. I have a "toggle" key for GPS etc on one of my screens and just toggle GPS off for a second then back on and it works.
David