It's hard to imagine Apple releasing new Mac Pros at keynotes, like they did in the past. In the Power Mac days, Apple would have those fun "bake-offs" with a Power Mac going up against a PC. It's hard to imagine Apple today, with all the attention it has on it and all the consumer mindshare, dedicating one of their beloved keynotes to what has now become an old-fashioned goal: making the fastest, most powerful PC/Mac on the planet.
Also, when Apple releases new products at keynotes, they tend to first go over the "state of things" at Apple, and I don't think they'd have any great news to share about their dedication to pro users in terms of software or sales of hardware. The presentations about the high-end software (Final Cut Pro, etc) were interesting to me, even though I don't use them, but it seems Apple is very much consumer-focused now.
I think, if there is a new Mac Pro, it will be a soft release on the web-site only, or an extra item in a keynote that focuses mainly on some other product.