"But i think if it's any decent kind of IBL support it's at least got the basics of final gathering and/or radiosity or something like that..."

Maybe it's a development of True Ambience (and the background process involved in that, which I suspect is some variation of final gather). The Sky Dome is one of the "light" sources that will illuminate surfaces in True Ambience mode. An image on the Sky Dome would have a similar effect.

If this is the case, then it seems possible that True Ambience will have been developed / fixed compared to the implementation seen in Bryce5. At the very least a proper fall-off level would be needed, and hopefully proper interaction with mesh smoothing and booleans.

Fully working True Ambience would probably be enough to make me buy Bryce6 at some stage.