> I guess it's because of the way that 1.9 'auto integrates' gems?
> 
>   irb19
>>> defined? Gem::RubyGemsVersion
> => nil
>>> require 'rubygems'
> => false
>>> defined? Gem::RubyGemsVersion
> => nil
>>> Gem::RubyGemsVersion
> => "1.3.0"
>>> defined? Gem::RubyGemsVersion
> => "constant"
> 
> After this point the rubygems_version method works in 1.9


I've seen this too; it's especially annoying as it stops rails from booting out of the box.

The fix is easy though. In boot.rb, change: 
	unless rubygems_version >= '0.9.4'
into:
	unless (rubygems_version || Gem::RubyGemsVersion) >= '0.9.4'

I'd be very interested in hearing the explanation for this weird behaviour!