###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

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