sqlite3 gem fails to install

I had an almost similar problem as you where I had sqlite3 installed and yet the sqlite3 gem would not install. I was seeing an error message as follows,

user-name-rajas-macbook-pro:rails3 user-namer$ gem install sqlite3Building native extensions.  This could take a while...    ERROR:  Error installing sqlite3:    ERROR: Failed to build gem native extension.        /Users/user-namer/.rvm/rubies/ruby-1.9.2-p180/bin/ruby extconf.rbchecking for sqlite3.h... yeschecking for sqlite3_libversion_number() in -lsqlite3... nosqlite3 is missing. Try 'port install sqlite3 +universal'or 'yum install sqlite3-devel' and check your shared library search path (thelocation where your sqlite3 shared library is located).*** extconf.rb failed ***Could not create Makefile due to some reason, probably lack ofnecessary libraries and/or headers.  Check the mkmf.log file for moredetails.  You may need configuration options.Provided configuration options:    --with-opt-dir    --with-opt-include    --without-opt-include=${opt-dir}/include    --with-opt-lib    --without-opt-lib=${opt-dir}/lib    --with-make-prog    --without-make-prog    --srcdir=.    --curdir    --ruby=/Users/user-namer/.rvm/rubies/ruby-1.9.2-p180/bin/ruby    --with-sqlite3-dir    --without-sqlite3-dir    --with-sqlite3-include    --without-sqlite3-include=${sqlite3-dir}/include    --with-sqlite3-lib    --without-sqlite3-lib=${sqlite3-dir}/lib    --with-sqlite3lib    --without-sqlite3libGem files will remain installed in /Users/user-namer/.rvm/gems/ruby-1.9.2-    p180@rails_3/gems/sqlite3-1.3.3 for inspection.Results logged to /Users/user-namer/.rvm/gems/ruby-1.9.2-p180@rails_3/gems/sqlite3-    1.3.3/ext/sqlite3/gem_make.out

Here is what worked for me

gem install sqlite3 -- --with-sqlite3-dir=/opt/local

I was following the Rails3 in Action book with rvm using ruby 1.9.2.p180 and rails 3.1.0.rc6 using Mac OS X 10.5.8 (Leopard) and when I ran:

rake cucumber:ok

I got:

Please install the sqlite3 adapter: `gem install activerecord-sqlite3-adapter` (can't activate sqlite3 (~> 1.3.4, runtime), already activated sqlite3-1.3.3. Make sure all dependencies are added to Gemfile.) 

I then did:

gem install sqlite3 -- --with-sqlite3-dir=/opt/local

Followed by:

bundle install

Hopefully, I will find my own answer on the Internet when sqlite 1.3.5 is required.

Sounds like you haven't installed XCode.