There are "99 problems" lists for Lisp, Prolog, and Haskell.
Is there something comparable for Erlang?
I'm looking for something that ascends from the very basic things to more complex, in ways idiomatic to Erlang. That is, list length or RLE compression is an obvious start. But then I'd like exercises in message passing, network and cluster stuff, etc, crafted by someone experienced.
erlang.org has a course with exercises specific to Erlang.
I guess the closest thing to what you say is the Erlang cookbook:
I don't know if that's what you are looking for, but I found something interesting on Project Euler. It's not Erlang.specific, but it could be a good starting point.
If you are looking for some good books and ways to learn Erlang, then it is answered here already.
If you're looking for programming practice on challenging programming problems in those languages I'd try Facebook Puzzles, Sphere, or Project Euler. I have found that Facebook Puzzles are most interesting, while Sphere online judge is most complete.
You might want to try rosettacode, which has a variety of programming problems in a bunch of different languages.
It looks like the problems that have been solved in Erlang are mostly the easy ones, but they might give you a start and it would also give you a chance to solve some of the other problems yourself. Since there are answers for so many languages, you might be able to look to other solutions for inspiration, as well.
Here you have the solution to 99 Prolog problems solved in Erlang.