I am currently running heckle 1.4.2 and rspec 1.2.4 to test Addressable. I recommend using the spec command directly to heckle anything rather than setting it up through a rake task. Heckling is something I typically do just before a release, rather than during regular development because it's so time-intensive. And when you're trying to track down surviving mutations, you're likely to want to go method-by-method.
Older versions of RSpec were much chattier — dumping the full spec results for all failing specs — but I sent through a patch that fixed that. Any of the recent versions don't have that problem, and heckling should work just fine.
For example:
spec spec/**/*_spec.rb --heckle Addressable::URI#normalize
Output:
**********************************************************************
*** Addressable::URI#normalize loaded with 25 possible mutations
**********************************************************************
25 mutations remaining...
24 mutations remaining...
23 mutations remaining...
22 mutations remaining...
21 mutations remaining...
20 mutations remaining...
19 mutations remaining...
18 mutations remaining...
17 mutations remaining...
16 mutations remaining...
15 mutations remaining...
14 mutations remaining...
13 mutations remaining...
12 mutations remaining...
11 mutations remaining...
10 mutations remaining...
9 mutations remaining...
8 mutations remaining...
7 mutations remaining...
6 mutations remaining...
5 mutations remaining...
4 mutations remaining...
3 mutations remaining...
2 mutations remaining...
1 mutations remaining...
No mutants survived. Cool!
(Results not typical.)