Why won't Bundler install vPim?

I have the following in my Gemfile:

clear_sources
bundle_path "vendor/bundler_gems"

source "http://gemcutter.org"
...
gem "vpim",                 "~> 0.658"
# a whol开发者_如何学JAVAe bunch of other gems

When I run gem bundle, I get everything installed except vpim. Running gem bundle --list | grep vpim returns nothing.

I'm using bundler08 because the project can't be upgraded to Bundler 0.9 yet.

I solved this by installing vPim manually (gem install vpim --version "~> 0.658") and then copying the gem specification to my bundled gems directory. Bonus points for anyone who figures out why Bundler didn't copy the specification.