You've always been able to get away with 1-2T past the official Shimano spec. If Shimano starts supporting 32T on their mid-cage derailleurs you can probably run a 34T cassette no problemo. Most OEM road bikes come with the short cage rear derailleurs so you may need to buy the mid-cage derailleur separately if you want to run shorter gearing.
Look for the P/N stamped on your rear derailleur. GS=mid cage. If you're not sure, just google the P/N to find your answer.
Last edited by Dunbar; 09-30-12 at 05:08 PM.