Shimano has gone through a lot of change lately in a relatively short space of time.
Dura-Ace is the top range shimano group. It can be had in standard mechanical setup or electronic Di2 which means different (and hideously expensive) derailleurs and levers. DA is followed (in order) by:
Ultegra (10 speed)
105 (10 speed)
Tiagra (9 speed)
Sora (8 or 9(?) speed)
2300 (or some number like that) (8 speed)
the main complicating factor is that shimano have just gone through a major update of their components (Dura-Ace, Ultegra and 105) for 2010 with new surface finishes, changed derailleurs, levers and brakes. the new levers are not compatible with the older front derailleur.
Its easy to tell a bike with 2010 components: the shifting cable is now hidden beneath the bar tape.
Im sure those manufacturers who bought components in bulk in the last few years will be mixing and matching whatever they can for the next 6 months or so.
Check out the tech archives at cyclingnews or similar to see what's changed lately. I cant be bothered keeping up anymore.