[UPDATE, 3 October 2012] The will_paginate gem has been updated since my original post and should now work with all recent versions of Rails (3.2.8 as of today). [end update]
Apparently, will_paginate was barely working with Rails 3.0, requiring users to specify a prerelease version of the gem that hadn’t been updated in ages. With Rails 3.1, even that no longer works, and will_paginate seems likely to be gone for good. Fortunately, RailsCasts has the solution! They suggest using kaminari, and so far so good. Not only has it not broken my app just by being installed, it’s also got some nifty features and is easy to understand.