What I didn't mention earlier is that Nathan's Yamamura Bike was included in The LEGO Book. If you have a copy of the book, have a look at page 192 (he is one of the Fan Builders they highlighted. This piece was also auctioned in the Brothers Brick "Creations for Charity" auction last year. The auction is coming up again soon, and is a rare opportunity to buy pieces from top designers, and help a worthwhile charity that gives toys (new LEGO sets) to children.
Details of the 2010 auction can be seen at this link: http://www.flickr.com/photos/nannanz/5065967369/
The Brothers Brick page says: "Creations for Charity 2010 is here! This is an annual fundraiser where builders donate MOCs to sell and raise money to buy Lego for underprivileged children. This is probably the only occasion where buyers can own the original Lego creations by diverse and talented builders (while funding a good cause too!). Last year we sold 30 creations ranging from custom minifigures to spaceships. We donated almost $3,000 worth of Lego to Toys for Tots!"
You can see the items and prices sold for from last year's auction at this link: http://www.flickr.com/photos/nannanz/4158818639/sizes/o/in/photostream/