The lightest-weight and lowest-profile nook case I have seen, and have tried, is the Industriell front cover:
http://gifts.barnesandnoble.com/Indu...?cds2Pid=35723
It's not the least expensive option out there, but I believe it is the most minimal. The quality is very good and I haven't had any problems with damaging the nook while carrying it in a bag with that case on.
Good luck!