If fitting a seatpost bike rack puts your saddle too high, then I suspect that you've either bought the wrong size frame, or still live in the mistaken belief that a rider should be able to have both feet flat on the ground while straddling the saddle.
And at the risk of getting flamed - that's a hipster's bike you've bought.
25% bike, 75% fashion statement.
It is not meant to be sullied by useful artefacts like racks, fenders, lights and such.
It "should" be combined with a Messenger bag, and you may adorn the bike by either hanging a big U-lock over the bar, or loop a heavy duty chain + padlock around the seat post.
Regarding a rack: It doesn't appear to have any fender/rack eyelets by the dropouts, and definitely none by the seat stays.
Your friends in need then are P-clamps:
http://www.bikecommuters.com/2008/07/19/p-clamps-are-your-friends/
Once you've sourced these little nuggets, go rack hunting. Maybe something like this:
http://www.jensonusa.com/store/product/RA308A08-Axiom+Transit+Tubular+Alloy+Rack.aspx
Do note that I make no guarantee that this particular rack will fit, as you will be trying to mount the rack a bit higher/more forward than what's originally intended.
This is one of those things best picked up in a shop, where you can easily hold the rack against the frame and see how they meet up.
