So I've just started working as the sole programmer at a non-profit 
which is heavily internet-based. Most of the code is in PHP or Perl, 
and I would like to gradually migrate it to Ruby over time.

I get a lot of leeway in this job, but this sort of a decision is 
probably the sort of thing I'd have to justify to my boss, who's 
likely to be nervous about it. Not about my ability to be more 
productive with Ruby, but with the fact that Ruby is an uncommon 
language compared to PHP and Perl. So if I get hit by a truck, or if 
the organization expands and needs to hire new programmers, or when 
we're looking for interns, it will be harder to find programmers who 
know Ruby.

Anybody have any advice as to what I can say him to make him less 
nervous about making such a transition?

Francis