A couple of speakers that may be good (I don't know if these guys are actually available for speaking enagements or not)...
Robert Hudson, the Adventures in Aquascaping Columnist from TFH
Jason Baliban, his name has come up over at the Aquatic Gardners Association Convention ( http://www.projectaquarium.com/about.aspx )
Also I'd be interested in hearing someone talk about Apistogramma spp. I think someone did this at the Cincinnati ACA last year, but I didn't make it that day to hear it.