I use Cocoapods for managing my Cocoa's project dependencies. I had an issue with the RegexKitLite. I got an 'unrecognized selectro send to instance' issue when I wanted to use a method declared in a category added by RegexKitLite in the NSString interface.
To fix it, add the flags '-all_load' in the Build settings under the 'Other Linker Flags' section.
More info : http://stackoverflow.com/questions/2567498/objective-c-categories-in-static-library