It's always been illegal to ride anywhere on the GW Parkway, from Mount Vernon to I-495. 36 C.F.R. 4.30(a) (compendium).
This really hasn't been enforced in years past, mostly because riders were not a nuisance, limiting use to off-peak hours, usually early on weekend mornings.
However, from what I've seen lately, many riders are hitting the GW Parkway after work during rush hour. This has caused a backlash from drivers, resulting in several recent op-ed pieces published in the Mount Vernon Gazette, (here and here) calling on elected officials to spur the U.S. Park Police into action.
Well, it seems like the always bike friendly) (sarcasm) U.S. Park Police have received the message. They are currently on heavy enforcement, handing out $500 tickets plus court costs to bicyclists on the parkway.
Not telling anyone what to do or not do -- just a head's up before your wallet is $500 lighter!
UPDATE: This week they have posted NO BIKES signs on the GW Parkway between Alexandria and Mount Vernon. Playing dumb just got a lot harder.
I saw a biker pulled over today getting a ticket but didn't have much sympathy since it was PM rush hour. It is that kind of behavior which has ruined this route for everyone.
With the neighborhood complaints, the politicians involved, the increased enforcement and the new signs, the Parkway seems too hot right now, imho.