This may be of no use to many out there, but I do know that there are a few people who have asked about mixing their Canon flashes with their studio strobes, say as a hair light or something like that. Well, I own a 550EX, and I recently bought a Sigma 500DG Super, and the Sigma has a feature that the Canon does not, and I didn't even know about it when I bought it. So, I thought I'd share. The Sigma can be used as a straight optical slave, i.e. it doesn't need the ETTL control information to cause it to fire. So, when in this slave mode, when your studio lights go off, your Sigma will go off as well. Pretty cool, eh? 
I don't think I'll be doing this myself, but it's at least nice to know about the option.
-Deva
