Had this happen over the weekend, the crank arm fell off mid ride. It's looking very much that the holding pins at the top weren't properly fixed during a service a few weeks back :( There's a narrow band of torque advised for attaching the crank arm; too tight and the pins can shear off under strain and too lose it'll just fall off. I suspect they've tightened it by hand rather than use a torque wrench.
Luckily it happened pootling down an unbusy cyclepath on a Saturday morning rather than on some of the excessively hard / fast runs I'd done the previous week. And there was a cab office just around the corner that got me home safely. The guy at the local bike shop was really apologetic and I'm glad it didn't turn into an argument about who's responsibility this was.
After having a bit of a read around about different crank sets I've ditched the FSA Gossamer and ordered a nicer Shimano CX specific one.
So basically I'm an idiot. Something vital falls off my bike because of a less than perfect service and I show them what's what by shelling out a load more money upgrading the thing they broke.