I've been using the region editor to generate some landscape textures rather than doing the whole worldspace by hand and it worked very well using the 'pineforest' and 'fieldgrass' textures.
However, using any kind of 'snow' or 'tundra' does not seem to work properly. The grass was generated nicely, with the textures all being somewhat blended together and creating a smooth transition between them.
The snow and tundra textures seem to be generated very roughly, completely filling some cells and not blending well. This is frustrating because I need to go through and fix nearly every cell that has been generated.

Has anyone else had this problem?
Is there a way to fix it without needing to hand-edit the whole world?

Do you have some screenshots? Normally such problems happen on the borders of your region, or because you haven't set up your region in the right way.

Oblivion (and most likely Skyrim) have a limit of 8 different textures that can be used in one cell. Propably you've been generating more than that?

http://img821.imageshack.us/img821/6135/10502115.jpg

There are only 6 textures being generated. Even with 3 or 4 it would be like this when I used snow or tundra textures. Green grassy textures worked very well, even when I generated 8+ just to test if the cell limit was affecting it.

I've fixed it mostly. It was being caused by the textures being only applied within certain height ranges. When a texture reaches the upper limit of the range it stops (as expected) but this can make some cells have messed up textures like this. The quads all need the same number of textures otherwise the remaining textures will be applied 'heavier' to make up for the one that has gone.
The easiest and fastest way to do this is to simply make all the textures have a high upper limit such as the default 200,000 and use increasing density amounts and lower limits for the textures that should appear higher.
I probably didn't explain it well, but hopefully it helps anyone else with the same issue.