trying to install rails 2.3.11

I've seen this before with a newer version of Rack

Can you post the output of "gem list"

Andrew

*** LOCAL GEMS ***

actionmailer (2.3.5)
actionpack (2.3.5)
activemodel (3.1.0)
activerecord (3.1.0, 2.3.5)
activeresource (3.1.0, 2.3.5)
activesupport (3.1.0, 2.3.5)
arel (2.2.1)
bcrypt-ruby (3.0.0)
builder (3.0.0)
bundler (1.0.18)
capistrano (2.8.0)
erubis (2.7.0)
highline (1.6.2)
hike (1.2.1)
hoptoad_notifier (2.4.11)
i18n (0.6.0)
mail (2.3.0)
mime-types (1.16)
multi_json (1.0.3)
mysql2 (0.3.7, 0.3.6)
net-scp (1.0.4)
net-sftp (2.0.5)
net-ssh (2.2.1)
net-ssh-gateway (1.1.0)
polyglot (0.3.2)
rack (1.3.2, 1.0.1)
rack-cache (1.0.3)
rack-mount (0.8.3)
rack-ssl (1.3.2)
rack-test (0.6.1)
rails (2.3.5)
railties (3.1.0)
rake (0.9.2 ruby)
rdoc (3.9.4)
rubygems-update (1.8.10)
sprockets (2.0.0)
thor (0.14.6)
tilt (1.3.3)
treetop (1.4.10)
tzinfo (0.3.29)
xml-simple (1.1.0)

I think its rack 1.3.2 that's causing the problem.

try running the following:

gem uninstall -v=1.3.2 rack

Then boot rails again.

Andrew

[21:03:52]kevin[ndthub]$ sudo gem uninstall -v=1.3.2 rack
Password:
INFO:  gem "rack" is not installed
[21:30:10]kevin[ndthub]$ ruby script/server
Missing the Rails 2.3.11 gem. Please `gem install -v=2.3.11 rails`, update your RAILS_GEM_VERSION setting in config/environment.rb for the Rails version you do have installed, or comment out RAILS_GEM_VERSION to use the latest version installed.

You're getting the error message because the rails server can't find Rails 2.3.11.

I see from your gem list that you have Rails 2.3.5 installed, along with parts of rails 3.1.0.

First thing I'd do is uninstall all the 3.1.0 gems (activemodel, activerecord, ...) with
   gem uninstall activemodel -v=3.1.0  # do the same for all the others)

Then I'd try the gem install again:
   gem install rails -v=2.3.11

Then do another gem list to see what you have.

*** LOCAL GEMS ***

actionmailer (2.3.5)
actionpack (2.3.5)
activemodel (3.1.0)
activerecord (3.1.0, 2.3.5)
activeresource (3.1.0, 2.3.5)
activesupport (3.1.0, 2.3.5)
arel (2.2.1)
bcrypt-ruby (3.0.0)
builder (3.0.0)
bundler (1.0.18)
capistrano (2.8.0)
erubis (2.7.0)
highline (1.6.2)
hike (1.2.1)
hoptoad_notifier (2.4.11)
i18n (0.6.0)
mail (2.3.0)
mime-types (1.16)
multi_json (1.0.3)
mysql2 (0.3.7, 0.3.6)
net-scp (1.0.4)
net-sftp (2.0.5)
net-ssh (2.2.1)
net-ssh-gateway (1.1.0)
polyglot (0.3.2)
rack (1.3.2, 1.0.1)
rack-cache (1.0.3)
rack-mount (0.8.3)
rack-ssl (1.3.2)
rack-test (0.6.1)
rails (2.3.5)
railties (3.1.0)
rake (0.9.2 ruby)
rdoc (3.9.4)
rubygems-update (1.8.10)
sprockets (2.0.0)
thor (0.14.6)
tilt (1.3.3)
treetop (1.4.10)
tzinfo (0.3.29)
xml-simple (1.1.0)
[15:06:03]kevin[ndthub]$ sudo gem uninstall actionmailer -v=2.3.5
INFO:  gem "actionmailer" is not installed
[15:06:20]kevin[ndthub]$ sudo gem uninstall activemodel -v=2.3.5
Password:
INFO:  gem "activemodel" is not installed

this is crazy. how can i just start over and remove all gems from the ruby version. If i install a new patch version of 1.8.7 that will allow me to start from scratch right?

rvm rubies

=> ruby-1.8.7-p352 [ x86_64 ]
   ruby-1.9.2-p290 [ x86_64 ]

ok, i was using rvm so i created a new gemset and installed a fresh 2.3.11 version of rails.