libicucore is a C/C++ library providing Unicode support.
iPhoneOS uses a legacy version of ICU. On firmware 2.x, it is version 3.6, and on firmware 3.x, it is 4.0.
References
- Official website: http://site.icu-project.org/
- API reference for version 4.0: http://icu-project.org/apiref/icu4c40/