They all have different "fingerprints" also:
Tony$ find / -iname libSystem.B.dylib -exec md5 {} \; 2>/dev/null
MD5 (/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.0.sdk/usr/lib/ libSystem.B.dylib) =
4a1f661aed002e958367b5dc13c09132
MD5 (/Developer/SDKs/MacOSX10.6.sdk/usr/lib/libSystem.B.dylib) =
c2bce3a788adc0c9c6f9c0653746464e
MD5 (/Developer/SDKs/MacOSX10.6.sdk/usr/libexec/oah/Shims/libSystem.B.dylib) =
d3ce7f45a2823046c0b952aa14e4533e
MD5 (/Developer/SDKs/MacOSX10.7.sdk/usr/lib/libSystem.B.dylib) =
50fd848fb976c1555aef7c8a8f95178c
MD5 (/usr/lib/libSystem.B.dylib) =
df8588f83c7d286c289e828847fab0d7