I've just had mine fixed. Basically my belt was wearing out so i had it changed. but the pulley was the reason it was wearing out - something that wasn't detected at the time. A couple of weeks later the belt snapped and I lost the power steering whilst driving - to cut a long story short, the belt was replaced under warranty but again within a couple of weeks the same thing happened. My mechanic - Oscar from McFarlane Automotive - a top chap and awesome mechanic - did some deeper checking and found that the pulley had worn out and was moving slightly which was causing the belt to slip off and/or shredding part of the belt.
Now i'm not sure if it's the alternator pulley, but suspect it might be because i got the battery charge warning on the centre display
Hopefully it's been sorted now but could be a major issue if this happens again, i'm keeping my fingers crossed.