GolfScript, 107 characters
.n?):L;'1'/{,}%{1$+)}*;][]\{:A{{+}+[1L.~)-1]%&}+1$\,.@^\[[[A]]+{|}*]+}/{.{L%}{%$..&1$,1$,/*$=}:C~\{L/}C&},,
The input must be given on STDIN.
Examples:
11
01
-
0
111
111
-
1
100
001
001
-
2
11100
10101
11100
-
1
101
010
101
-
5
