there seems to be an issue with postgresql; my project uses the postgresql gem and when trying to run your query_memcached, it drops an initializing error:
parasew:content-machine parasew$ script/server
=> Booting Mongrel
=> Rails 2.3.3 application starting on http://0.0.0.0:3000
/Library/Ruby/Gems/1.8/gems/activesupport-2.3.3/lib/active_support/core_ext/module/aliasing.rb:33:in alias_method': undefined methodexecute' for class `ActiveRecord::ConnectionAdapters::MysqlAdapter' (NameError)
from /Library/Ruby/Gems/1.8/gems/activesupport-2.3.3/lib/active_support/core_ext/module/aliasing.rb:33:in `alias_method_chain'
from /Users/parasew/Documents/GIT/content-machine/vendor/plugins/query_memcached/lib/query_memcached.rb:87
from /Library/Ruby/Site/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
from /Library/Ruby/Site/1.8/rubygems/custom_require.rb:31:in `require'
from /Library/Ruby/Gems/1.8/gems/activesupport-2.3.3/lib/active_support/dependencies.rb:156:in `require'
from /Library/Ruby/Gems/1.8/gems/activesupport-2.3.3/lib/active_support/dependencies.rb:521:in `new_constants_in'
from /Library/Ruby/Gems/1.8/gems/activesupport-2.3.3/lib/active_support/dependencies.rb:156:in `require'
from /Users/parasew/Documents/GIT/content-machine/vendor/plugins/query_memcached/init.rb:2:in `evaluate_init_rb'
... 17 levels...
from /Library/Ruby/Gems/1.8/gems/rails-2.3.3/lib/commands/server.rb:84
from /Library/Ruby/Site/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
from /Library/Ruby/Site/1.8/rubygems/custom_require.rb:31:in `require'
from script/server:3
update: when removing line 73-89 from query_memcached.rb (the class MysqlAdapter) it works. strage behaviour. maybe you have to explicitly overwrite mysql or postgresql.