Source vs Drain polarity for MOSFETS
To clarify a bit what others have already said, a MOSFET has a internal diode that points from source to drain in N channel devices and drain to source in P channel devices.  This is not something added deliberately by the manufacturer, but is a byproduct of the way MOSFETs are made.  Most of the time this diode would prevent the MOSFET being useful when flipped around.  There are some applications you can consider "advanced" where this diode is actually used deliberately.  One example is to make a synchronous rectifier.  That's basically a diode with a transistor accross it.  The transistor is turned on when it is known the diode should be conducting.  This lowers the voltage drop accross the diode and is sometimes used in switching power supplies to get a little more efficiency.  A MOSFET with its internal diode can be thought of as the diode and transistor all nicely integrated into a single package.

Your observation of source being negative and drain positive is true for N channel FETs.  Just like there are NPN and PNP bipolar transistors, there are N channel and P channel FETs that are mirror images of each other polarity-wise.  A P channel FET would be connected with a positive source and negative drain.  In the off state, the gate is held at the source voltage.  To turn it on, the gate is lowered by 12-15V with respect to the source for most normal MOSFETs.