I haven't figured out what prompts it to do this, but I also permanently leave it on, and what happens for me is basically this:
you'll tap the shutter and the camera will come awake, everything shows up in the lcd and viewfinder. you try to AF and you'll feel it sort of go down but there won't be a reaction from the AF. you'll try to take a photo and it won't depress. only thing to do is turn it off and restart it, and then its fine.
the sort of thing that just scares the bejeezus out of you the first time because you think you've broken the thing altogether