In fairness to Leno, he doesn't have the same flexibility as Letterman when it comes to negotiating directly with the writers.
Letterman's company, World Wide Pants, is able to negotiate a separate agreement with the writers because it is his company, and not part of CBS. The Tonight Show, on the other hand, is produced directly by NBC, and therefore it isn't able to negotiate it's own agreement with the writers, but is stuck with whatever negotiating tactics are employed by the parent company.
That said, the fact that Letterman was able to reach an agreement with the WGA certainly suggests that the WGA is more than willing to negotiate in good faith. Now that World Wide Pants has shown that an accomodation can be reached, the big media companies need to get off their tails and get back at the table with the union reps.