The devs did some testing... nothing broke on the tests that already worked, a bunch of new tests were enabled, and about 3/4 of those passed. Most of the differences between GL 2.0 and 2.1 seem to be in the common code above the individual HW drivers.
Please not that r600c have supported most 2.1 extensions for a long time. This mostly (only?) adds GLSL 1.2, and not all parts of GLSL 1.2 is quite working yet.
So this patch makes some, but not all, programs requiring GLSL 1.2 work, but if an application where to have two code paths, one for GLSL 1.0 and one for 1.2, and the 1.2 code path used something from the non-working stuff, this would break it.
That being said, I know of no application this breaks, and wine seams to like it, so I'm happy...