It took me a few hours to sort this all out on my own on Friday, but now that something like an "official" guide for Ruby developers has been released, I'm glad to find that I ended up choosing the preferred path!

After recompiling Ruby 1.9 as a 64-bit binary, installing the 64-bit version of MySQL, and getting the rest of my gems back in shape, things worked quite well.

That said, I'm not looking forward to spending several days helping people out with upgrades at work, haha. This one was quite a bit more involved than I expected.

NB: Had to do the same for Python's MySQL bindings with MySQL-64.