In the enterprise I work for, there's more employees than there are desks to seat them all. This is resultant of a number of factors: frequent need for people to work at different locations, people that telecommute regularly, and just an overall short-fall of available office/desk space.
For the most part, this hasn't been a problem for me. If anything, it encourages me to work from home (and, I tend to get more done while working from home, at any rate). Unfortunately, when we have our meeting days, it causes a larger number of people, than normal, to concentrate in the meeting facility. This tends to create an acute shortfall in desk space. While I have an assigned seat in the building where our large-group meeting is held, most attendees don't. So, if I arrive other than ass-early the day of the meeting, I frequently find my desk occupied by visitors.
On days where I'm not going to be busy, elsewhere in the building (typically, when I'm stuck in one of the labs), I politely evict the visitor. If I know I'm going to be elsewhere most of the day, I typically just set my stuff in my cube and let the visitor continue borrowing my desk. Generally, it's not a problem - if I'm not using it, I don't care, overly much, if someone else does.
That said, there's always people that seem to feel compelled to abuse the courtesy. Most frequently, the abuse are merely annoying. Generally, it's limited to my stuff being moved out of their normal places or paperwork/trash being left behind. While annoying, it's not a big deal. What is a big deal, however, is when people "borrow" stuff from my desk (note to people who borrow desks: if you find things like plastic silverware, blank CDROMs, pens, etc, in the desk, those are not consumables there for use by all an sundry. I pay for the shit sitting in my desk and your use of it is petty-theft).
Oh... I also always love when visitors take it upon themselves to make themselves at home at my desk by adjusting my seat to their personal comfort. It's oh so lovely to come back and find my chair's settings completely jacked up. It's also nice when one of the visitors has been eating at my desk and left food debris ground into the chair's fabric.