I'm sure it's probably happened but I'm equally sure it's happened with other frame materials too. To be honest, despite the fact that I'm not a big fan of Al frames, I have yet to see one fail during the normal life of a frame. The only times I've actually seen an Al frame fail was on MTBs that have been subjected to jumps and hucking.

BTW, here's a pretty good webpage that compares various metals used in bike frames.