The USCF is planning to roll out another rating system, for Blitz events, starting March 1st. The rule update document (link to PDF) outlines the new rating:
Blitz rating system:
Blitz has its own rating system. To be rated, the time controls require from 5 to 10 minutes total playing time. See also 5C. Ratable time controls, and have a minimum primary time control of 3 minutes. All rounds must use the same time control. Hence G/3 inc/2 is rated under the Blitz system. Currently Lightning and Bullet are not included in any rating system.
The following (also found in the rules update) is a useful overview of the various time controls and which rating system they fall under.
5C. Ratable time controls. There are three rating systems. Regular (slow), Quick (fast) and Blitz. For the purposes of rating G/minutes and inc/seconds (or d/seconds), add minutes (mm) and seconds (ss) for total playing time for each player. (i.e. total time equals minutes plus (seconds times 60)) or mm+ss; e.g.: G/60 d/5 = 60+5 = 65 minutes total playing time for each player. Multiple time controls add all mm for each control: mm = mm(1) + mm(2) + …
Regular only: total playing time for each player is greater than 65 minutes. mm+ss > 65
Dual (both regular and quick): total playing time for each player is from 30 to 65 minutes. 30 < mm + ss < 65
Quick only: total playing time for each player is more than 10 and less than 30 minutes. 10 < mm+ss < 30
For Regular, Dual and Quick the primary time (mm in minutes) must be at least 5 minutes.
Blitz: total playing time for each player is from 5 to 10 minutes inclusive and the primary time control must be at least 3 minutes. 5 < mm+ss < 10 All rounds must use the same time control.