Ok, this wasn't documented anywhere, (the best I could find was the Rails 3.0 announcement and the Bundler rationale) so after reading the Rails guide I hacked around with it on my own.
In the old days (Rails 2) you could do something like this:
and it would take the required gems and copy them into vendor/gems.
But Rails 3.0 gets rid of this handy feature in favor of Bundler's approach. Bundler's basic approach assumes that you'll always run bundle install in every deployment environment and that it can go to the internet to get the gems. Neither of these assumptions are valid in my case.
There is one way of doing this:
bundle install --deployment
but that copies everything, rails, ruby, 10,000 gems I've never heard of (well ya, I've heard of 'em, but sheesh) and nicely bloats the target environment. So it seems it is all or nothing.
Anyone else figure this out?