For anyone who is interested in an inexpensive long term airport parking solution near IAD (location is the Hampton Inn Dulles Airport South in Chantilly), Groupon's Washington DC deal of the day (valid unitl midnight 6/20/2011) is for up to 14 days' parking for $20 with free shuttle bus between the hotel and IAD. You can buy and combine two groupons if your trip is longer.
Here is the link to the main Washington, DC groupon page (I don't think I can post a direct link to the offer without using my personal referral code, which I know is frowned upon since there is a groupon CONGA on the SPAM forum):
http://www.groupon.com/washington-dc/.