Bob, we try to spend as much time as we can manage on the forums, but if you need a guaranteed response, you'll want to
email our support ninjas.
We haven't got a particular ship date for OmniGraffle 6 in mind yet, and the upgrade pricing/policies are some of the last things we work on. If you buy a license now, it'll be good for all 5.x releases, but beyond that we can't really say.
When considering a purchase like this, one always needs to weigh the cost of buying against the productivity you're giving up by waiting. If your current version of OmniGraffle meets your needs, waiting is a perfectly valid choice.
Our upgrades have always been constructed in such a way that folks who skip a version of the app aren't penalized for doing so - they get the same discount. Our thinking there is that folks should only ever have to buy the new version if they think they'll use it, not in order to hold on to some maximum discount percentage.
Amusingly, the above policy generates occasional angry emails from the folks who *do* buy every release, and are irritated that folks who didn't do so get the same discount! I don't know why they want us to punish you, but we promise we won't. ;-)