My take on the pedal power meter status @ Interbike, in order appearance in market:

1. Look/Polar: most expensive, proprietary head unit communication protocol (not ANT+, may go to low-power BlueTooth at some point in future), force detection only (requires pedals be carefully aligned, uses magnet for approximate cadence determination)... no accelerometers. Fully functional units have been distributed, so close to available. Demo'ed in Interbike w/ Polar head unit... not sure if open road tests were available.

2. Garmin Vector (formerly Metrigear): Presently Look-compatible pedals, but technology is applicable to almost any pedal if they decide to expand (early prototypes were Speedplay, but Speedplay wouldn't allow for good warranty support). ANT+ Sport protocol (good with many head units). Uses accelerometers for "instant cadence" and automatic orientation determination (if pedal thread orientation changes, it will figure it out eventually). Electronics are inside spindle instead of on spindle as w/ Polar (may make for stronger or lighter spindle, not sure if this affects accuracy). Demo'ed in Interbike on trainer w/ Garmin head unit.

3. Brim Brothers: cleat-only solution, using cleat adapter plate for Speedplay, so won't work with 4-hole Speedplay-specific shoes. Disadvantage is cleat position relative to propulsive force direction is constantly changing, which requires accurately and instantly tracking that, while the spindle orientation relative to force position only changes when threads are rotated. Demo'ed in Interbike on trainer w/ Garmin head unit. Trainer tests are easiest, though, because vibrations make it much harder to determine cleat orientation from accelerometers.

Various companies are looking at crank-arm based power meters, and these would also work for tandems if a tandem set were designed.