Perhaps they have spent the time implementing features which even more people have asked for? Look at the dates on the recent posts to this thread and remember that there are a lot of copies of OG out there; when something comes up that is a real deal-breaker for many customers, the posts come at shorter intervals than half a year :-) I agree that it sucks to have a specific need that (relatively) few others have the good sense to also need and thus get largely ignored by the development schedule. On the other hand, if what you are requesting really is a popular request, it is hard to imagine that they will go off and do other things that don't have much support, unless they feel it is something important to the product and the customers haven't realized they need it yet.
To file an official request, or to ask a question and be guaranteed a response from someone at Omni, use Help->Send Feedback from within the app, or email
OmniGraffle@omnigroup.com (the former is better if you are making a bug report or the like, as it includes license and version information in the subject line automatically, and I recall reading somewhere that messages so tagged get priority processing).