I'm playing newer Snapshot (13w36a) and see hills covered by snow.
Maybe add functionality to select to which biome convert %biome under discussion% based on final generated height?

I don't know details will minecraft itself generate snow on high blocks or not, but if it will, this topic does not matter.

New vanilla list of biomes (keeped for me):
0 - Ocean
1 - Plains
2 - Desert
3 - Extreme Hills
4 - Forest
5 - Taiga
6 - Swampland
7 - River
8 - Hell
9 - Sky
10 - FrozenOcean
11 - FrozenRiver
12 - Ice Plains
13 - Ice Mountains
14 - MushroomIsland
15 - MushroomIslandShore
16 - Beach
17 - DesertHills
18 - ForestHills
19 - TaigaHills
20 - Extreme Hills Edge
21 - Jungle
22 - JungleHills
23 - JungleEdge
24 - Deep Ocean
25 - Stone Beach
26 - Cold Beach
27 - Birch Forest
28 - Birch Forest Hills
29 - Roofed Forest
30 - Cold Taiga
31 - Cold Taiga Hills
32 - Mega Taiga
33 - Mega Taiga Hills
34 - Extreme Hills+
35 - Savanna
36 - Savanna Plateau
37 - Mesa
38 - Mesa Plateau F
39 - Mesa Plateau

Minecraft will use a system where the snow height is based on the biome temperature: lower temperature causes snow to fall on lower elevations. It won't replace the biome as far as I know.

The issue with replacing biomes based on height is that if the player can't extend/destroy mountains, snow won't fall on the correct columns anymore. (It still snows on a sea level if there was previously a mountain there.)