Scrub it out with a nylon scrubbie or brush. If you dried it well, then maybe it's just food particles that didn't come off. If it's rust, then it wasn't dry.

Scrub it good, use salt for extra grit if needed. Dry it well, best done by setting it on a warm burner for a while.
Wipe it down with shortening before storing.

Pre-seasoned cast iron is NOT the same as a well seasoned cast iron - it's just a very thin coat and you'll need to make sure the pan is dry and greased until it's got more seasoning.

Leah,

There's a thread going on about just this thing over in the cookware forum:

http://ths.gardenweb.com/forums/load/cookware/msg0918171717144.html?5

Check that out plus this guy's link:
http://www.richsoil.com/castiron/

Everything you could ever want to know! :)

Good luck!
Jane

Here is a link that might be useful: Cast iron conversation

After cleaning it you're supposed to take a paper towel dampened with olive oil and wipe your cast iron pot with that.
That prevents the rusting.
I've heard that it's very healthy to cook with those, they leave a trace of iron in your food.

Here is a link that might be useful: everythingschooling

Go easy on the shortening or oil applications...you don't want your skillet "swimming" in it. Also, watch the pans or skillets closely while on the burner drying. It's easy to get distracted & forget about them til the smoke alarm goes off. (trust me!)