I might add it in the full 3rd edition. The problem is that rvm and rbenv can't coexist on the same machine, so the barrier to using both is high. In particular, it means that I haven't been able to test rbenv myself, so I don't know it. I'd probably have to install, say, a Linux virtual machine just to use it.

That said, if you or someone you know would be willing to write up some material on rbenv to parallel the rvm instructions, I'd be happy to include it in the book. Please send any correspondence to admin at railstutorial dot org.