Here's something that's been kicking around for a while.
It's a twin top tube frame from Subrosa. Not scheduled for production as yet, but it's definitely out there and being riden by Justin Kosman.
It's different and you'd have to say "retro", but definitely not an engineering advantage ie. it's not stronger or lighter than a regular frame. In fact it is either a) weaker or b) heavier.
But at least it's something different. Credit for that.
It looks pretty nice from this angle, but looking down of a split tt with gussets and braces and things might not look so clean. Check this thread on BMX forum about the frame.