It's an interesting use of GoogleMaps. Whether it's worth anything to attendees is yet to be proven. For exhibitors, viewing the attendee distribution via the map should allow them to staff appropriately with the applicable regional reps or distributors, if that's an option.
Not to bust anyone's chops, but technically this isn't a mashup, which involves integrating two technologies. What the VoIP event is doing is creatively using GoogleMaps.
If you want to do the same for your event, you can figure out GoogleMaps or just start here.
If you're interested in seeing the VoIP attendance distribution, here's the map (note - it may take a long time for all the little flags to load).