The poll results are in: after nearly 90 votes, it's RSpec in a landslide. A full 76 of 87 respondents (87.36%) prefer RSpec for use in the Ruby on Rails Tutorial book. This is good news for me, since I know RSpec better, and I also have the specs through Chapter 6 already written. For those who voted for Test::Unit, I hope you'll come along for the RSpec ride; even though it's not currently the default, I believe RSpec is the Rails Way*, so these results are good news for you, too.
To make the introduction to RSpec a little gentler, I'll plan to add some more introductory information in Chapter 2, perhaps including a separate box on RSpec or a section on "anatomy of a spec". If you still run into any RSpec confusion, please let me know, and I'll do what I can to alleviate it.
*The Shoulda testing framework is a good alternate choice—the Other Rails Way, as it were.