This only happens to me on Ruby 1.9.3, in Mac OS X Lion (10.7.3). 1.9.2/1.8.7 are not affected by this issue.
After building ruby-1.9.3-p0 OR ruby-1.9.3-p122 (via rvm), I get the following issue attempting to require 'digest/sha1' (or md5, etc).
1.9.3p122 :001 > require 'digest/sha1'
LoadError: dlopen(/Users/me/.rvm/rubies/ruby-1.9.3-head/lib/ruby/1.9.1/x86_64-darwin11.3.0/digest/sha1.bundle, 9): Symbol not found: _rb_Digest_SHA1_Finish
Referenced from: /Users/me/.rvm/rubies/ruby-1.9.3-head/lib/ruby/1.9.1/x86_64-darwin11.3.0/digest/sha1.bundle
Expected in: flat namespace
in /Users/me/.rvm/rubies/ruby-1.9.3-head/lib/ruby/1.9.1/x86_64-darwin11.3.0/digest/sha1.bundle - /Users/me/.rvm/rubies/ruby-1.9.3-head/lib/ruby/1.9.1/x86_64-darwin11.3.0/digest/sha1.bundle
from /Users/me/.rvm/rubies/ruby-1.9.3-head/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in require' from /Users/me/.rvm/rubies/ruby-1.9.3-head/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in require'
from (irb):1
from /Users/me/.rvm/rubies/ruby-1.9.3-head/bin/irb:16:in `'
Weird. It's happening on both of my laptops, but not a fresh one. I guess something is screwed up from when Lion first came out. Time to format... (go ahead and reject)
I found the source of the bug. My ~/.profile defined $C_INCLUDE_PATH and $CPLUS_INCLUDE_PATH. Having this here did not affect Ruby 1.8.7 or 1.9.2. It did, however, screw up portions of 1.9.3. How strange.