Your version of PHP was not compiled with the iconv extension enabled. You'll need to contact your host to fix this, or if you're running a VPS or dedicated server, you'll need to recompile PHP with the iconv extension.
For reference, if anyone is unable to install/use the software because of a lack of iconv, contact us and we'll do what we can to help.
It is proving to be an important requirement, so the test scripts now reflect this. It is worth noting that it is normally enabled by default in PHP5, so there has to be an explicit choice to disable it.