Washingtonian recently did a click-baity feature on the "9 Coolest Developments Coming to Washington in 2017" and two of them include key sections of bike trail.
The first is the Wharf. In addition to inspiring normally nice Iowans to wish a recession on our town, the massive development on the SW Waterfront is also building a section of the Anacostia Riverwalk Trail. The trail project was long ago broken up into 19 pieces, and the Wharf section is #17. The portion completing this year, phase 1, will only build part of that; but when complete it would leave slightly less than 5 segments to go (part of 18 opened last year).
Status of the ART in 2012
The ART will be right along the waterfront (which is not the Anacostia waterfront, but who's keeping track)
Cyclists will likely prefer Maine Ave, which will feature a protected pair of bike lanes, behind parked cars and at the sidewalk level - a first for DC. That will also not be completed in this phase. Phase 2 will be completed in 2020-2021.
The other project isn't really in Washington, but rather Riverdale Park, MD. The Riverdale Park Station development is built atop an old trolley line and portions of that right-of-way from Hyattsville to College Park have been turned into a trail. The RPS property is the only gap. Though part of this project will open this year, it's not clear the trail will.
Interestingly, as far away as these projects are from another, you can ride from one to the other entirely on trail (except for the gap between the Wharf and Nationals Stadium).