"==" is an equality check that calls various methods[1] to answer the question. "is" is true if and only if the two references are actually the exact same reference, or, under the hood, basically that the two pointers are equal.
>>> [1,2,3] == [1,2,3]
True
>>> [1,2,3] is [1,2,3]
False
The equality check in the case of lists is recursive and checks the equality of its elements piecewise. But they are two different lists, so they don't check "is" the same.
As an optimization, because everything in Python is an object (despite occasional claims to the contrary by the Ruby community), certain objects are created at interpreter start time and cached forever, such as (IIRC) the numbers 0 through 100 or so, so if you happen to use "is" it will sometimes "work":
>>> (1 + 3) is (2 + 2)
True
but it's not good or reliable:
>>> (1000 + 3000) is (2000 + 2000)
False
Basically, unless you are doing something with "object identity" and you know what you are doing (and if you don't know what I mean by caring about "object identity" you probably don't pass this check), OR are checking that something is specifically "None" and not merely false (as None is another of those cached values and there is only ever one None in the system), don't use "is".
[1]: http://docs.python.org/reference/expressions.html#notin