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):

"string" in["string"]