Originally Posted by
Little Darwin
I believe that the 28 is as large a cog as you can use with a 10 speed Shimano.
Shimano can do 10-speed beyond 28T, you just need a MTB-type derailleur (e.g., Deore) to handle the larger cogs and greater capacity, and a wider-range 10-speed cassette. Found an 11-32T and 11-34T 10-speed cassette at Harris Cyclery, but they're expensiive ($160):
http://sheldonbrown.com/harris/k7.html#10
Your setup of 50/34T and 11-28T is probably plenty low enough for most people though, except for loaded touring.