I am running an Ultegra SL drivetrain...
I am running an Ultegra SL drivetrain (RD-6600G on the back, I believe) with a CS6600 11-25 on one wheel set and 11-23 on another. I would love to put a CS6700 11-28 on one but have heard/read that my rear derailleur can not handle the 28? Is this true? Do I have to hunt down a CS6600 12-27?
28 tooth should be fine-rated cog capacity is usually correlated to what size cassettes the manufacturer is offering at the time. Shimano's largest cassette cog offered was 27 tooth when the 6600 series came out. With the 6700 series they added the 28-tooth option. More info. here under capacity: http://sheldonbrown.com/gloss_ca-g.html.
Write your answer here...I think I had the same set set up as you and I moved to Europe. There are a lot of steeper grades over here, so I threw on an ultegra 12-30 and it worked great.
I think I have the same set up, and thew a ultegra 12-30 on and it works great.