Starting from iPhoneOS 3.1, all default libraries are combined into a big cache file in /System/Library/Caches/com.apple.dyld/dyld_shared_cache_armv6 (or _armv7) to improve performance. The original files are no longer useful for non-on-device-developers, so they are eliminated from the system.
Cache Extraction
Developers who does not use the SDK cannot link programs on the iOS directly due to the missing dylibs. One first need to extract the appropriate dylibs from the dyld_shared_cache. One could use dyld_decache by KennyTM~ to extract these dylibs.
Alternatively, one could also use DySlim by comex to mount the whole cache file on Mac OS X. decache by phoenixdev also works quite well.
References
- Cache or Check? — an analysis of the dyld_shared_cache system by D. Howett.