In the end it boiled down to finding the proper require statements -- I'm not sure if this is the best way though, so any comments are appreciated. Here's how you do it:

#!/usr/bin/ruby
require 'rubygems'
require 'active_support/inflector'

puts Inflector.singularize('inflections')

Explanation

I've found that you need to require 'rubygems' first, which is to be expected (as ActiveSupport, at least on my system, is a gem). But then it becomes unintuitive.

Because if you then attempt to require_gem 'activesupport', the Inflector isn't made accessible:

$ irb
irb(main):001:0> require 'rubygems'
=> true
irb(main):002:0> require_gem 'activesupport'
=> true
irb(main):003:0> Inflector
NameError: uninitialized constant Inflector
        from (irb):3

...and you can't require_gem 'active_support/inflector', because that's not a valid gem:

irb(main):004:0> require_gem 'active_support/inflector'
Gem::LoadError: Could not find RubyGem active_support/inflector (> 0.0.0)
        from /opt/local/lib/ruby/vendor_ruby/1.8/rubygems.rb:204:in `report_activate_error'
        from /opt/local/lib/ruby/vendor_ruby/1.8/rubygems.rb:141:in `activate'
        from /opt/local/lib/ruby/vendor_ruby/1.8/rubygems.rb:37:in `require_gem_with_options'
        from /opt/local/lib/ruby/vendor_ruby/1.8/rubygems.rb:31:in `require_gem'
        from (irb):4

...but you can do this instead (and it only works after loading the activesupport gem):

irb(main):005:0> require 'active_support/inflector'
=> true 
irb(main):006:0> Inflector
=> Inflector
irb(main):007:0> Inflector.singularize('inflections')
=> "inflection"

Even more confusing is the mixing of activesupport and active_support path names, but I assume that's the difference between the gem's name ("activesupport") versus its location on disk (.../active_support/).

I really should RTFM.

Update 2006-06-29: I helped!