Originally Posted by Chris_W

I'm hoping that the comment about the new 10-speed MTB stuff not being compatible with 9-speed MTB stuff means that if you change to a 10-speed cassette, you'll need a new chain, new rear shifter, new front derailleur (because the chain is slightly narrower), and Shimano want you to get new chainrings (although that probably won't be necessary).

The comment about the new 10-speed MTB stuff not being compatible with 10-speed road stuff hopefully ONLY means that a road rear derailleur won't work with the MTB cassette (just like was the case for 9-speed stuff), but everything should be OK if you use a MTB rear derailleur.

The only way that this wouldn't work is if Shimano made the cog spacing or chain width different for their mountain and road components, which I doubt they will. I also assume that the new rear derailleurs use the same geometry and so require the same cable pull as the old ones; since Shimano hasn't changed this for a long time, I don't expect that this to be the case, and even if it is then you would still be able to use one of the older ("9-speed") MTB Shimano rear derailleurs without a problem with the new cassettes.

So, in summary, the incompatibility that Shimano states makes complete sense without making it difficult to use these new 10-speed cassettes with an otherwise road setup (except for MTB RD).