Besides being fun to race and, at times, annoying to deal with, Metro buses have one other noteworthy quality for cyclists. You can take your bike on them at any time, on any line. You can carry folded bikes on board or place bikes (except tandems or other odd-shaped bikes) on the racks on front.
Sadly the same is not true of all other transit systems. Several bus systems are without racks. Most trains don't allow normal bike access (everyone allows folding bikes - though some require that it be in a carrying case). These system should re-evaluate their policies and equipment to allow for more bicycle access during more of the day.
Every bus system should have racks. In fact, Congress should mandate that transit buses sold in the U.S. come equipped with racks. Trains should make more allowances for bikes, including expanding hours and providing hooks or set aside space. Future transit systems, such as the new light rail lines, should include space for bikes.
Here are all the local transit systems I could find ranked first to worst (in my opinion).
Metro Bus - 100% Bike rack equipped, no restrictions
Ride-on - Montgomery County Transit - Ride-on is 100% bike rack equipped
ART - Each ART has a bike rack capable of transporting two bikes
DC Circulator - They have bike racks, no information on the website though (OT:the "dining" section is "under construction" - ouch, nothing screams unprofessional like dead-end links)
Fairfax Connector -
No information on their policy available, but pictures don't show racks. Update: They now provide bike racks and can be considered Good.
CUE (Fairfax) - Currently bicycles are not allowed on CUE buses. Update: As of July 2009, bicycle racks have been installed on the front of all CUE Buses
Metro Rail - Blanket rule forbidding non-folding bikes for 6 hours a day(rush hour) - no exceptions.
Amtrak - Some lines allow for bikes, but the service "varies widely from train to train, and station to station" and you have to call to find out if you're OK. They charge a fee, which isn't constant and don't allow for unusual sized bikes. They do allow folding bikes. I feel like Amtrak could make some money running bike cars to Harper's Ferry, Cumberland and Pittsburgh on Friday evenings (and back from Harper's Ferry and Cumberland on Sundays) allowing cyclists to ride one way on the C&O and Allegheny Highlands trails, but I'm not sure if that train has space for bikes.
UM Shuttle buses -
No information on their policy available, but this site seems to indicate that there are no bicycle racks Update: UM Shuttle buses are being equipped with bike racks but not all of them are yet.
Update: Potomac and Rappahannock Transportation Commission (PRTC) - "Bike racks have been installed on 40 PRTC buses that serve the OmniLink local routes, the Cross County Connector route and the Metro Direct routes that connect to nearby Metro stations. At this time, PRTC does not anticipate installing bike racks on OmniRide commuter buses."
VRE - Full size bicycles cannot be accommodated on VRE trains. Collapsible bicycles, however, are permitted on all of our trains. Bikes will be boarded and removed last—after all pedestrian passengers have boarded or detrained. Update: As of May 2009, Bicycles are allowed on the last three northbound trains, the midday and the last three southbound trains of the day. VRE is limiting bikes to two per train car and says they must be tethered to train seats.
Georgetown Metro Connection - No information on their policy available (but this is such a small connection that it may never be used) Update: defunct
DASH(Alexandria) - No bicycle racks available. No information on their website. Update: Bike racks were added in February 2013
MARC - Collapsible bikes only.
George (Falls Church) - No information on their policy available. Defunct.
The Bus (Prince George's County) - No bicycle racks available. No information on their website. Update: In May-June of 2015, bike racks were finally added
MTA isn't on the list because it's Baltimore, but
You are welcome to take your bicycle on MTA Light Rail and Metro Subway trains.
All MTA buses are not presently equipped with racks; however bike racks are now available on selected routes.
The best part...
If the train is crowded due to evening or morning rush hour, sporting events, or special events, please wait for the next train.
As near as I could find, no one has been injured by a bike on MTA light rail or Subway.
Addendum: According to several comments, UM Shuttle buses are being equipped with bike racks but not all of them are yet. So we'll upgrade it to "Bad" until it reaches 100%.