If you got errors like this:

willy@extensa projects $ padrino –help
/usr/lib/ruby/1.8/rubygems.rb:280:in ‘activate’: can’t activate bundler (= 0.9.7, runtime) for [“padrino-core-0.9.1”], already activated bundler-0.8.1 for [] (Gem::LoadError)

After upgrading Bundler:

willy@extensa projects $ sudo gem update bundler
Updating installed gems
Updating bundler
Due to a rubygems bug, you must uninstall older versions of the bundler gem for 0.9 to work.
If you still need bundler 0.8, install the ‘bundler08’ gem.
Successfully installed bundler-0.9.9
Gems updated: bundler

It is easier to update all your gems, but for gems still using 0.8 like appengine-tools, here are my brilliant steps :)

1. Install bunder08
2. Search for bundler occurences:
   cd /var/lib/gems/1.8/specifications
   (wherever your path is..)
   egrep "<bundler>.+?0\.8" *
3. Replace only the first occurences of them
   (all of them to be safe if you’re using ancient Rubygems)
4. Uninstall bundler 0.8