My wife drives the car more than I do, so not even in jest is something like that ever going on.
I bet they would sell a lot more if it were a magnetic decal.
2011 Sapphire Black/Black E70 X5 M "Torquey"
2010 Jerez Black/Black E90 M3 DCT -Gone-
2009 Montego Blue/Grey E90 335i -Gone-