I'm a recent college graduate on the job hunt. Many companies ask me
what my college GPA was...My GPA isn't very impressive at all -- so I
lied about it. Is there any way that employers can call up or find out
what my GPA was?

The employer could ask you to sign a release authorizing the school to
give the employer your grades. That would seem to be a pretty standard
thing to do.  Try playing it straight, explaining why you're so much
better than what is reflected in your grades.  Quote the President
about the importance of grades. Tell the employer your school
misunderestimated you. May s/he'll have a sense of humor.
   Deception is rarely worth the psychic cost.  And you *could* get caught.

You may get away with it for a time but it would be always hanging over you.

Look on the bright side, some employers may prefer a less academic
candidate. Who knows where the interviewers are coming from?

Excellent comment anonoboy!  But you left him hanging:
"To the C students, I say, 'You too can be president of the United
States,"' Bush said at the 300th commencement of Yale University.
FoxNews
"Self Deprecating Bush Talks to Yale Grads," (Associated Press, May 21, 2001)
http://www.foxnews.com/story/0,2933,25229,00.html

A Google search strategy using the following will find some links with
Pres. Bush's SAT scores and at least part of his Yale transcript. 
Needless to say they aren't NORMALLY Yale standards:
President Bush quotation grades

Best regards,

Omnivorous-GA

I recently graduated too and was in the same boat as you. Employers
usually ask for a transcript of some sort. If they don't then they'll
check GPA as often as they do references. I've never had a job call a
reference to ask questions about me, most are too lazy to check. If
you really want the job or really need it, it might not be worth the
risk. If you don't have a chance anyway put down a better GPA and say
that you thought they wanted major or non-major GPA if they ask or
something akin to that.

You don't have to put down your Overall GPA.
Have you calculated your GPA for your Major?

Sure they can : 
- By requesting to see transcripts and other documentation supporting your claim. 
- By hiring someone to investigate your past, if the job is really
important, or job security is really important.
- By applying polygraph or psychological tests.