Canon will support their flashgun technology in all future cameras, as they do with their lenses, some of the technology that works is pretty much hidden from everyone else so some things that may be in the pipeline will probably not work with aftermarket gear. Sigma lens are a good example, they work fine with current bodies but struggle when a new body is released.
Build quality is another issue with some aftermarket stuff.
Not sure if the Jessopp gun acts in exactly the same way as the 580ex, for instance does the Jessopp act as a controller for other flashguns, the 580 can be hotshoe mounted, switched to fire only an activating beam for a slave and take no part in the actual exposure. There are a host of other things it will do, will the Jessopp also do them?
Matt