Why would a manufacturer change chainring sizes drastically between years? A friend was just showing me the 2010 Cervélo S2 specs, as well as the 2011 S2 specs, and the Ultegra line, at least, changes from a 53/39 chainring set in 2010 to a 50/34 set in 2011. This seems like a pretty steep difference, especially given that the more expensive Red and Dura-Ace lines for 2011 keep the larger chainring sizes. 

I can't imagine someone spending $4,000 on a bike being that put off by a larger chainring set  than someone spending $5,000 enough to cause customer demand to change the size.
Well, in 2010 there's the S1, S2 and S3.  S1 is lower-end and has 50/34.

In 2011, they dropped the S1 and there's only the S2 and S3.  (based on what's up on their website right now)

The lowest end of the line is probably considered something like "entry level" or at least "not for racers", so it gets the "slower" chainring setup.  They've kept the chainring size on the cheapest bikes in the line the same, they just moved the labels for what the cheapest bike is.

And it's probably all about marketing and whatnot, nothing to do with any real solid technical reasons.