“The last two years with Cervélo at CSC was perfect. Not just the bike, but the people behind the bike was really important. They follow you around in all the races and test a lot and they listen to the mechanics and the riders and that’s really important.” -
Henk van Lijsdonk, Team CSC mechanic, to pezcyclingnews.com.  

In other words Cervelo receive feedback from riders across a wide spectrum of abilities, from relative novices to the pros of Team CSC and are never content to stop looking for any possible improvement in their already excellent framesets.
The RS frame was developed as a result of positive feedback from both novice riders and CSC professionals using the Cervelo R3. The idea was to produce a new version of the R3 with a higher handlebar position to suit a wider range of rider preferences.

The RS is designed to accommodate these more upright positions while maintaining the high performance characteristics that set Cervélo apart, and so Cervelo have the R3 and Soloist models for those with average or above average flexibility, and the RS for those who prefer a higher handlebar position.

The RS does not fall into the trap so often found in the bike industry of equating a higher handlebar position with less of a desire for high performance. From tube shapes to carbon lay-ups to stiffness & strength, the RS is so good that some CSC riders will use it next year, while those pros and regular riders with average or low handlebar positions will continue to be perfectly served with the R3 and Soloist ranges.