hey so i spent some time looking into a folding fixed gear at one point but then decided not to undertake the project. i was on crags list today and some one is selling a folding Raleigh from the 70's. Sheldon brown has one of these converted to a fixed gear on his web site....if any one is new york is interested in undertaking project this might be a good start... let me know how it works out:

http://newyork.craigslist.org/mnh/bik/707235879.html