It seems Formtastic depend on a Rubyforge-library that been dead since early 2008 (ouch). There's a fork of that project that lives on GitHub that been very active. Maybe should be considered to try that one out; seems like it's faster as well as it replaced Hpricot for Nokogiri (and the spec suit is slow as f*ck now so...). Seems it was polished a lot, added helpers etc.
Go for it!
Mailed the dude behind, trying to get him to claim the project and get it on gemcutter. Might take some time, but no hurry really.
Update: I forked that project as "rspec_tag_matchers" and it now lives on Gemcutter after some modifications. On Ruby 1.8.7 specs run 4 seconds faster. B) For some reason specs run slower on Ruby 1.9 (wierd), but "rspec_tag_matchers" is 1-2 seconds faster.
Note: Hey, in 4 seconds you can run and push the coffee machine button (I noticed)! ;)