I slipped with a probe, shorted a power to ground, and burned a trace off my protoshield (sparkfun variant) today. Seems the first 7mm or so of trace from the +5v pin is the fusible link in the design.
I'm a bit surprised. I was running on USB power and I thought those were limited to 500mA. Perhaps PCB traces don't handle as much current as I thought.
So, for people making protoshields, I'm glad the shield failed before the Arduino, but...
It would be nice to have a resettable fuse to limit the current to something under the trace failure current. I looked at polyfuses but wasn't able to find one of an appropriate current limit that didn't have an unacceptably high resistance.
An alternative would be to beef up the +5 traces so they could handle the 1 amp or so that the onboard regulator makes, and hope the USB won't make more. It is currently a very narrow trace so it can sneak through the hole field. A wider trace would have to go around, but could be serveral times wider... heck, have both.