Anyone know a good place to ride one of these. From the way last winter went I'm beginning to think this is going to be as close to snowboarding as I'm going to get around here

How about Rock Creek Park?

Try Beach Drive on a Saturday, or Sunday when it is closed. Park near Military Road and ride the section from Military to Broad Branch rd. This section is all downhill and is over a mile long. :-)

http://www.nps.gov/rocr/ppmaps/rocrmap%2Epdf

For real mountain terrain, any mountain which allows mountain biking should be OK with mountain boards. I suspect Timberline would allow them on the Salamander Trail--a trail open to MTBs and also used as an access road for staff on ATVs. If you want steeper, you'll probably have to brave some single track--there are some excellent trails just above Frederick in the Catoctins.