DX10 hardware - such as the GeForce 8800 or the Radeon 2900 - won't work with the new DX 10.1 features. The 0.1 revision requires completely new hardware for support, thus royally cheesing off many gamers who paid top whack for their new hardware over the last few months on the basis of future game compatibility.
But these gamers shouldn't fret too much - 10.1 adds virtually nothing that they will care about and, more to the point, adds almost nothing that developers are likely to care about. The spec revision basically makes a number of things that are optional in DX10 compulsory under the new standard - such as 32-bit floating point filtering, as opposed to the 16-bit current. 4xAA is a compulsory standard to support in 10.1, whereas graphics vendors can pick and choose their anti-aliasing support currently.
The spec is likely to be ill-received. Not only does it require brand new hardware, immediately creating a miniscule sub-set of DX10 owners, but it also requires Vista SP1, and also requires developer implementation.