stir plate

I'm making a yeast starter using a stir plate for the first time. It's been running for a couple of hours and I'm just curious as to whether the yeast is going to foam, or is the circulation of the liquid going to keep that from happening?

yes, you will get some foaming. Depending on how much head space you have in your flask it may even foam out the top.  I suggest using a 2000ml flask and not filling it more than about 1400ml so you have some space. Also cover the top loosely with a piece of aluminum foil or something so that as the stir plate is running it can allow oxygen in. You definitely don't want to run your stir plate with a fermentation lock on top.

You will probably get less foam with the stir plate.  The stirring action agitates the wort forcing more co2 out on a continuous basis.  I have stopped the stir plate once for a couple of hours, when I started again there was a big blow off of co2 and a lot of foam.  When I run the stir plate continuously I don't get a large head build up.

David

As stated above. The magnet that stirs the yeast breaks up any bubbles that would form. If you still worry? Use a drop of (fermcap) to eliminate the chance of foam forming. Positive pressure will push out oxygen and keep a layer of C02 in the flask. Use sanitized foil on the top!

I get less than 1/2" foam when I use my stir plate.  I use a 2000ml Erlenmeyer with about 1500 - 1800 starter depending on the gravity of the wort.

I use a 3 liter flask.  I found that sometimes I see foam, but it's not as violent as you would see in your fermenter.  Because of the continuous movement of the stirbar, the CO2 dissipates.  Heck you might not even see the krousen(sp?).
Just my 2 cents.