I feel like contributing some more opinion to this thread since I live in western Kentucky where we too have mountain lion "sightings".
I have a hard time delieving it was a 220 pound cat, like someone posted, that is a world record size cat in an area that doesn't really have any. If it was a pet, that may be a good reason for it to be oversized though. As we all know from having lazy housecats, when they stop hunting and such, they get fat and lazy, heh. But I know of no one who really has a pet mountain lion.
I do not doubt that a mountain lion can sustain itself in southern Illinois or western Kentucky, but how in the hell did it get here? It would have had to have wandered hundreds of miles to get here. Somewhere along that way, it would have found suitable living conditions and stopped there.
Any animal spotted for real was probably someone's pet of some sort.
Like someone else mentioned too, if you guys are worried about a cat or two, don't go out west. On a side note, I did just read that article about the Canadian singer attacked and killed by coyotes, that sure is a freak incident.
http://www.cnn.com/2009/SHOWBIZ/Music/10/29/canada.singer.killed/index.html