Ok, maybe "boom" is going too far, but it's nice to see so many teams moving into venues that allow them to become profitable (not renting out NFL stadiums) and provide a nice, intimate atmosphere.
The stadium that started it all: Columbus Crew Stadium - 22,500 - 1999
The Home Depot Center
Teams: LA Galaxy and Chivas USA
Capacity: 27,000
Built: 2003
Pizza Hut Park
Team: FC Dallas
Capacity: 21,193
Built: 2005
Bridgeview Stadium (naming sponsor TBA..likely LaSalle Bank)
Team: Chicago Fire
Capacity: 20,000
Under Construction
Harrison Stadium (naming sponsor TBA)
Team: Metrostars
Capacity: 20,000
Completion in 2007

Earlier design
Colorado Rapids Stadium (naming sponsor TBA)
Team: Colorado Rapids
Capacity: 20,000
Completion in 2007
Also worth mentioning...
Rochester (USL, "minor league" team) has a 12,000 seat stadium u/c right now.
None of them are exactly cutting edge design, but they mark a significant point in the story of professional soccer in the U.S.
It should also be noted that they are all designed with expansion in mind (in the open ends, additional upper decks, or temporary seating on the stage ends).
New stadiums are also being planned in Salt Lake and D.C., but nothing has been finalized.