The version of Ruby that ships with OS X 10.8 (Mountain Lion), which is patchlevel 358, seems to have a bug that breaks Alaveteli.
On my machine at least, running /usr/bin/ruby -e 'puts 2**97' prints 0, rather than a big number.
I’ve just merged Henare’s patch that configures rvm to use p370, which seems to be okay.
It would be interesting if anyone else could confirm this bug. In Alaveteli it manifests itself as an error wrong number of arguments (1 for 0) on alaveteli/commonlib/rblib/util.rb:18:in `to_s'.
Cheers,
Robin