Case insensitive dictionary search?

I can use map to implement the case insensitive list search with Python.

a = ['xyz', 'wMa', 'Pma'];

b = map(string.lower, a)
if 'Xyz'.lower() in b:
    print 'yes'

How can I do the same thing with dictionary?

I tried the following code, but ap has the list of ['a','b','c'], not the case insensitive dictionary.

a = {'a':1, 'B':2, 'c':3}
ap = map(string.lower, a)