If you're quite familiar with Ruby/Rails, but you left the ecosystem for a while (before 1.9 existed), would it be better to re-acquaint yourself with 1.8/Rails 2 first, or just dive into the new stuff?
When developing new Ruby stuff, one thing to watch for is interoperability with old gems that are not 1.9-ready. But the good news is, most gems work just fine under 1.9 -- at least in the non-Rails world. I think I recall only one or two gems I tried to use that weren't updated to work with 1.9 yet, out of many that I use or discovered.
That said, if you're willing to do that, Ruby 1.9 is quite nice and a bit more performant as well.