Workstation GPUs are tending to be very simialr to gaming ones these days, but with extra support and features turned on via the drivers (often in conjuntion with the card). This has, in the past, made it possible to hack a GeForce series card into reporting itself to the drivers as a Quadra.
The additional features tend to be specific to the Workstation market, i.e. hardware anti-aliased lines for CAD usage.
More or less. This is hardly the only area of the computing industry where very similar products are sold with different labels at different price points. CPUs are another good example. If the manufacturer is getting better than expected yields on their CPUs (i.e. more dies on the waffer happily run at higher speeds) it is common to mark some as a lower clock speed despite the fact that they will run faster (currently the Athlon 3200+ Venice core CPUs appear to be able to run at least as fast a 3500+s).