!!Edit: This was written when 1.6 was first released. The steps to reproduce were for that version. However, the situation still applies to nearly all biomes (non-variation of passive mob herds during "sequential" new chunk generation).
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I noticed plains biomes are spawning disproportionately large numbers of sheep groups. The amount of cows and horses seems to be "normal" as well as chickens, maybe, but pigs have become a rarity.

I've been creating and recreating worlds with my very first ever seed and noticed the pig problem the other day when I set off in search of them after my food bar got low. I went to a nearby plains area where pigs are known to frequent and found herd after herd of sheep. I thought it might just be a coincidence that only sheep had been spawning in the area surrounding the spawn point so I flew around and around in a wider circle and still, only herds of sheep were spawning - pigs are very rare.

What I expected to happen was...:
Herds of pigs spawn with their usual random weight.

What actually happened was...:
Mostly sheep spawn all over the place.

Steps to Reproduce (particular to my world seed):
1. Create a new world
2. Named it "World 1"
3. Set to "Creative"
4. Set seed to <5437694978402696571> (brackets excluded)
5. Set to "Large Biomes", Structures: ON, Cheats: ON
6. From the world spawn point, head south until you reach a large lake, then head east into the nearby plains biome. Here is where I found spawn after spawn of sheep. Even flying all over the plains biome nearly all the spawns were sheep.

See also: attached crash report listing mostly sheep and a few chickens

Additionally: when working with Minecraft MCP on a personal local mod I noticed the random number generator was rolling an unusually high count of the number 11 (which apparently coincides with sheep in the plains biome).