Anybody else had this happen?
Mine is about 4 years old, and has led a pretty pampered life. So it came as a surprise to me a few weeks ago when I pulled it out that the CL and spring-loaded flip-down diffuser had fallen out of the head and were laying in the bottom of the flash compartment of my bag. I can't figure out a way to get them back inside there without actually taking the head apart, which I would prefer not to do.
Should I just send it in for repair (and pray the standard out of warranty repair fee on the 580 isn't outrageous) or is there something I've missed?
It doesn't look too complicated, but the mkII has weather sealing the 580ex doesn't. There is a switch shown in the diagram that senses if the panels are out or in and there's no telling if it was damaged.
Canon charges a flat rate for repairing flashes, about $130. Whether or not that's worth it would depend on whether you ever use the panels and if you ever plan to sell it.
FYI - Canon changed the shape of the contact pins on the foot from rounded to cone shaped to fix the problem some had with intermittent contact. Some early 580exII also had issues with arcing to the shield around the flash tube that darken the tube, especially with extensive HSS use. So if you do decide to send it it you should ask Canon to upgrade the foot and check the tube.
This happened to me, and your story sounds eerily familiar. I think the cheapest place to find the part was actually from Canon, though I don't remember the link (and couldn't spot it with a quick Google). I did take it apart and fix it myself. It was a few years ago, so I don't remember the details specifically. Mine works fine, so alls well that ends well. Perhaps there's a youtube video, who knows. I'm not sure it's worth doing it, but I imagine it does help resale value if you are successful and ever intend to sell it.
It hit me last night going to sleep why I ended up ordering and replacing this item: if the wide panel diffuser is not retracted, the zoom on the flash is automatically set to 14mm and can't be manually changed. That was enough to make me take it apart. So you won't be able to get a focused flash that will throw further, in theory.