MOSFET symbol - what is the correct symbol
It is likely that you saw a Circuit Lab sysmbol and that this caused you to ask this question. The Circuit Lab N Channel MOSFET symbol is both unusual and illogical.
I'd avoid using them if at all possible.
Read on ...

Acceptable [tm] N Channel MOSFET symbol tends to have these characteristics.  

Gate symbol on one side.  

3 "contacts" on other side vertically.
Top of these is drain.
Bottom of these 3 is source.
Middle has an arrow pointing INTO the FET and the outside end is connected to source.
This indicates that there is a connected body diode and that it is non conducting when the source is more negative than the drain (arrow is same as would be for a discrete diode).

Any symbol which obeys these guidelines should be "clear enough" and OK to use.
I have very occasionally seen people use a symbol which does not comply with these guidelines but which is still recognisable as an N Channel MOSFET. 

SO. Any of these are OK, and you can see the differences for the unmarked P Channels.

Many more examples here





But!!!

Jippie's example shows the rogue version.
[Note: See below - this is in fact intended to be a P Channel sysmbol].
Truly horrible. I'd have to wonder if this was a P Channel symbol or an N Channel one.
Even the discussion it is taken from has people expressing uncertainty re arrow direction. As shown IF that is an N Channel then it is implying body diode polarity and NOT current flow in source.  

Thusly



________________'

Circuit Lab is apparently the (or a) culprit.
This is their symbol for an N Channel MOSFET.
A nasty piece of work, alas. 
Arrow shows usual drain-source conduction direction BUT as a MOSFET is a 2 quadrant device and will provide a true resistive on channel with \$V_{gs}\$ positive BUT \$V_{ds}\$ negative, the arrow is meaningless and, as it is in the opposite direction to most N Channel MOSFET sysmbols it is misleading to most. (Note the proper use of this symbol in table below). 





USER23909 helpfully pointed out this page -  Wikipedia -  MOSFET . This page includes the following symbols. User xxx says these may be IPC standards, but Wikipedia is silent re their source.

Wikipedia MOSFET symbols



http://en.wikipedia.org/wiki/MOSFET#Circuit_symbols