Paul writes about the fact that Synapse IS (dammit) an ESB.
Back when we were originally pulling Synapse together, the intent was indeed to build something that multiple ESB vendors (hence the involvement from Sonic/IONA) could use as a mediation framework without needing to replace their entire products - cooperate on infrastructure, compete with higher-level products. While this idea had some merit, the fact is that it's quite challenging to define the boundaries between the "mediation" portion and the "registry" portion and the "endpoint management" portion. Add to this the distinct lack of actual coding participation from the extant ESB vendors, stir, let sit for a couple of years, and indeed you end up with a full-on ESB.
Whatever that is. :)
Regardless, Synapse has evolved into an awesome product, and stands head-to-head with (if not above) a lot of the commercially available ESBs out there. Viva la open-source!