The only exterior difference that i know of are the new rear lights (they have new style "spirals" inside - especially noticeable when lights are switched on). There are also a few minor changes inside like new door handles - you won't actually notice them unless you've driven (ot at least sat in) both versions of the car.
Other than that, the ordinary 5-series has got a new transmission and front bumper, but these are unchanged on the M5...
2007 Audi R8
2009 Audi R8 V10 - deposit down
2007 Audi S5 - gone