I posted about this in the MYOG forums, but I'll post here also.
There seem to be mixed results from the WPB cuben. Richard's testing didn't show a lot of promise for it, but anecdotal impressions have indicated "pretty good" breathability, which yeah, is vague.
The only things I made with the material when I had some included panels of 3-layer eVent, which could be explaining the reason why they seem to be working out fine.
I decided to stop bothering with the stuff as it was just too hard to nail down it's performance, coupled with a pretty huge price tag, poor test results, and less than concise data from cubic regarding the specs. However, afaik, they were working on refining it, so this material could have changed significantly, or not at all.