Linker Error in Xcode 4.2 Developer Preview
You should link libz.dylib not any specific version. The correct version gets used automatically.
Update the libz dylib version in your project definition to whatever your XCode 4.2 is providing. If you are compiling the same code for different targets, check the library version on each of them.
Since SDK 3.2 all versions of libz are symbolic links to the last version (libz.1.2.3 in SDK 4.3), so regardless of what libz-version you choose, you get the last one. See /Developer/Platforms/iPhoneOS.platform/DeviceSupport/Latest/Symbols/usr/lib.
I don't know what Apple will do in the future. If you link to libz you are likely to keep linking to the latest version. A change in the major number of the library (libz-X) could break compatibility, so a safer choice is to keep linking the same version you have now. Your choice.