Apparently Cervelo spent a lot of time making a lighter version of one of their frames (R5 IIRC?), the difference was so minimal they ended up adding thicker paint to the original to increase its weight and therefore enhance the weight difference.
Nonsense. The standard R5 (which I ride) is one of the lightest framesets in the world (depending how you weigh it). The R5CA, or project California is what I imagine you're talking about, and it was built to develop even better technology, regardless of cost. That technology has trickled down to the 2014 R5, but there's no way Cervelo would pointlessly add weight to a frame they sell 10s of thousands of in order to bolster the reputation of a bike they sell a few hundred of!
As has been said, you do get what you pay for with carbon frames, having ridden and owned quite a lot, I'm happy in the knowledge that my R5 is superior enough to a Ribble or Planet X to justify the additional expense.
If you're an occasional rider, you might not notice the differences as much, but if you race, you'll know which frames are better suited to different disciplines.
I also have a Scott CR1 which is a great bike for circuit racing, but if I was doing a century ride, I'd choose the Cervelo every time - the increase in comfort is incredible.
I had a Giant TCR for a couple of years and whilst it was a solid, comfortable frame, it lacked the stiffness of the Cervelo and the Scott.
The comment above about a really sorted frame making it feel like you're cheating rings very true!