I have a student loan through MOHELA (Missouri Higher Education Loan Authority) that has been open since 2004. I pulled my credit report a few weeks ago and it was showing that I had one 30-day, one 60-day, and one 120-day late reporting on it from fall 2005 (this account is one of three baddies on my report, including a paid medical collection from last year and an unpaid CO from 2005). I'd never heard of a GW letter before I found this forum, but I thought I'd give it a go. I did some searching last night and I found an annual report PDF that listed the email address of MOHELA's executive director.
I sent him an email around 1:00 a.m. EST last night and I got a voicemail about an hour ago from someone at MOHELA named Belinda (very friendly!) who said that a request has been sent to all three credit bureaus to remove the late payments from my history and that a letter to that effect is in the mail to me today. She also said that it can take up to 30 days for the change to show on my credit reports and to please give her a call if I have any questions.
WAHOO!!!!!!! I'm so excited!!!!!!! Now I might actually have the energy to start working on getting the other two baddies removed, as well. 
Thanks, Nanz!
Also, I just logged into my email and saw the following two replies to my initial emailed request:
Reply from the executive director at 12:16 p.m. EST today:
Amanda:
Thank you for your request. Yes, we will definitely be happy to accommodate your request. Most importantly, it is great to hear that you completed your degree. I have passed along your request to Mary _____, our Director of operations. The change will be made promptly, however, I am not sure how long it will take to make its way completely into all of the databases of various credit reporting agencies. Mary or her staff will respond to you via email when this has been completed. I wish you the best of luck in purchasing a new car!
Reply from Mary, the director of operations, at 12:48 p.m. EST today:
Amanda:
Your request has been completed. Normally it takes approximately 30 days or so for the various credit reporting agencies to update their database. We also sent you today a letter outlining the updates made for your records.
If you should need anything further, please let us know.
Thank you!
How nice and prompt are they?!? 
Congrats!! I'm going to try a GW for my SLs as well. Even if they don't remove all of the lates, maybe they will remove some.
Sure do! I'll PM it to you.