Initially Cervelo came out with the R and S series which were designed to be aggressive. The idea was if you needed the handlebars higher, you could always simply add spacers. But riders don't like spacers: they don't "look pro". Of course if the rider in question isn't pro, "looking pro" shouldn't be a concern. But for some reason people want to "look pro" for their club rides. And indeed there's a limit to how many spacers you probably want. Better to have the frame design utilize available space a bit better.
So they came out with the RS, a more relaxed geometry. No problem: now they spanned the geometry range. Now if you wanted the bars sufficiently higher than the R-series, you could get the RS. The RS also had longer chainstays for a bit more comfort over rough roads.
Yet some people didn't want to be seen riding the "RS", obviously a "fatty master's bike". It didn't matter how few spacers you had: just the letters "RS" looked un-pro.
So Cervelo for 2011 simplified: increase the chainstays up to the RS-standard (and the standard of the rest of the industry: the 2010 R and S series chainstays were exceptionally short), and more assertively increase the head tube length with the frame size. As a result, the large and extra-large R-series frame this year is basically the 2010 RS.
But the key difference is the frame says "R", not "RS". So riders need not hang their heads in shame. They're on the "pro" bike.
They also reduced the pedal-to-front-wheel overlap on the smaller frames. Some riders didn't like it that if you turned your wheel away from your forward foot, the tire could strike your shoe. The obvious solution is to not turn your wheel into your forward foot: to turn the wheel that much requires walking-pace speed anyway. It's not hard to avoid. But people don't like it, anyway. Cervelo's view was that good handling at high speed (which comes with the shorter front-end) is more important than handling at walking pace (which is compromised by toe overlap), but they've finally backed down from that position.
Here's a plot of the head tube length plotted as a function of reach, reach being the horizontal distance between the seat post and where the fork steerer tube exits the head tube. You can see the 2011's are all-around taller in the head tube: fewer unsightly spacers! The difference goes from 8 mm in the smaller frames up to the full gap between the 2010 R and RS in the larger frames.