[antubbs]

So, it's certainly happened:

http://en.wikipedia.org/wiki/2009_satellite_collision

I'm nowhere near talented enough to talk in terms of "odds" with respect to satellite collisions, but it's worth noting that certain orbit geometries are going to naturally see a lot more density because several satellites want to be in a somewhat similar orbit for various reasons.

[swatkat]

Another one:

http://www.space.com/20146-russian-satellite-space-junk-thre...