According to Minecraft Wiki, sheep have a 0.174% chance to be pink.[source] However, with this particular seed I managed to see a pink sheep next to my spawn point. I also found another pink sheep after some travelling. Talk about probability! The seed is this:
5657907404137179727
I tried this seed several times with the same result. Click the "read more" for more screenshots, coordinates, and info.
I originally found this seed at Minecraft Forum. Several users have said they have had similar results as well.
Here are some more pictures. Click them to see larger version.
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| Near initial spawn point. x=-100, z=240 |
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| Another pink sheep at x=70, z=70 |
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| A village at x=30, z=125 |