Bike Friday tandems and some Lightfoot Cycles options just use slip together sleeves:
http://lightfootcycles.com/products-...ustomizations/
There have also been some cool separables on VS, and a few simpler homebrews can be found on blogs here and there. For example, a nice example of a pseudo Herse coupling system:
http://www.flickr.com/photos/bensond...7601730455819/
And a not-so-pretty, but very creative demountable 20:
http://www.outsideconnection.com/gal...pv/takeapart1/