So here's what I did, and which I think you shouldn't have any issues replicating..Mind you, I wasn't contracting. I was working full time for a F500 company but we used to always work with clients at client locations and hence this could very well be applicable to you..
My designation in my firm was Senior Consultant. However my role title was Project Lead. So in my resume, I put it like this..
May 2008 - July 2012...............F500 CompanyJun 2010 - Present
...................Client ABC
.............................................(Senior Consultant) Project Lead
May 2008 - Dec 2009
...............Client XYZ
............................................(Consultant) Team Lead
In your case, it would look like this..
May 2009 - July 2012...............IT Contracting CompanyJun 2010 - Present
...................Firm ABC
...........................................(Contractor) Project Manager
May 2009 - Jun 2010
................Firm XYZ
...........................................(Contractor) Team Lead
Sorry for the messed up alignment but I hope you get the idea..You have to write your contracting firm as your employer, there's pretty much no way around it. However the above approach will help anyone seeing your resume get the sense that you have indeed worked with some big names in the market. Plus don't stress yourself over this. There are several folks from IT Contracting backgrounds that go through the process every year, so the ad com will pretty much understand what you're trying to convey.
I disagree with the poster above who advised writing something about this in your optional essay. I think this is too trivial a matter to be included in the optional essay. Keep your word space in that essay free for something else that may come up..
Hope this helps!!