[sdfjkl]

you can copy a virtualenv from your build environment to your production environment by just copying the directory

Not if you're developing on a different platform/architecture than you're deploying to (e.g. OS X vs. Linux). Unless you're only using pure Python modules, which excludes PIL and most database drivers.

Other than that, you're very right.

[scottyallen]

Yes, right. However, your build/staging environment should be the same platform/architecture as your production environment, for lots of reasons other than just this. Development environments can be different, of course...