Python (17 11):

b in a in b 

(Checks if b is contained in a and a is contained in b, if that wasn't clear from the code.)

Alternative python: (8 7)

derived from Tom Verelst's Go solution:

b in[a] 

Bonus: this works for any type.

EDIT:

Wait a second, just read that you can also directly program in the strings, and don't have to count quotes... (or at least, that what golfscript does). So... Python on par with golfscript? Oh my!

Alternative alternative Python (5 4):

(thanks Claudiu)

"string"in["string"] 

original:

"string" in["string"] 

Alternative Alternative Alternative Bendy-ruly Python (2):

"string"is"string" 

Nothing was said about comparison keywords (This is not a serious submission, just something that occurred to me...)