Sometimes with remote server's you may have to supply the files relating to the current version of rails your writing for in order to run your app on someone else's server, like mosso.com.
So in Terminal, while in your Rails apps directory run:
This will unpack the rails version your using into the /vendor/rails directory of your app.
From there you can upload it to your server and it should just work.
To reverse the process...
This removes the frozen version from your vendor/rails directory.
Picking Up Changes
Also note new rails versions come with config file changes, to pick these up run:
Freezing to the Edge
You can also freeze (or grab a local development copy) of rails via...
Or Freeze to a specific release with...
rake rails:freeze:edge TAG=rel_2-0-2
This freezes to Rails 2.0.2
You can also do this for individual gems, by first grabbing the gemsonrails gem...
sudo gem install gemsonrails
Then moving to your Rails app directory and running...
This installs some extra RAKE tasks into your app, so now you can freeze a local copy of say the RedCloth gem into your app via...
rake gems:freeze GEM=RedCloth