I know them well, we get the nexus hub here too

Shimano made one for the downhill scene but it never made production. But they make a variant for hybrid/ touring bikes, but the daddy is the rohloff. One of my pals has one of these...
http://www.nicolai.net/products/e-frames/e-nuc-tfr.html
and if you want a laugh, look at the FRAME ONLY price!
I have fancied one for years, but the weight over the back end of the bike is the issue... but the internals are nuts!
Not sure on the nexus but unlike deraulliers you can shift under full load on the rohloff, and most imprortant is the duplciation of gears with a 3x9 setup. the rohloff still has the same range with smaller increments. The negative bit is the price they are REALLY expensive. going to be £800 minimum.
And you should have a look at truativs Hammerschmidt plantery gear on the FRONT.
http://www.magicmechanics.com/home.p.../how_it_works/
popular for MTBs as it gives you the ground clearance
All wonderful but seriously expensive. Singlespeed is simplicity!