I'm even more confused now that I just found this on Shimano's website:

Increased line-up of Compact Cranks
21/09/2006
Nunspeet, The Netherlands
Shimano will add two compact cranksets to the range, following the introduction of FC-R700 last year. The new FC-R600 is a 10-speed compact crankset (50-34T). It has an integrated bottom bracket for increased rigidity and lightweight design. FC-4550 is a 9-speed compact crankset (50-34T) at Tiagra level, also with an integrated bottom bracket.

These compact cranksets feature a super smooth shifting performance, thanks to Shimano’s HYPERDRIVE technology. No need for another front derailleur or shifter. Just mount the crankset, adapt the chain length and you’re ready to go! FC-R600 is compatible with all Shimano’s 10-speed component groups while FC-4550 can be used with all 9-speed groups.

These compact cranksets will be available in three crank arm lengths: 165, 170 and 175mm. They feature a bolt circle diameter of 110mm.