People need to be very careful how much time they spend on the internet at work. In today's times of recession, and lots of people being layed off, it's not inconceivable that employers (particularly if they have a hunch/know if for example someone's a serial facebook user etc), that they won't ask their IT department to investigate when they're choosing which employee to make redundant.
It's happened to someone I know, and possibly ended their career in a profession they spent several years qualifying for at Uni, as no one else is hiring.