Somehow this doesn't work out of the box – is there an adapter I can buy that will allow me to connect the Displayport output of my Macbook to the Thunderbolt input of my 27" Thunderbolt monitor?
As @bmike, @djacobson, and @jaberg have stated in comments, that will not work. There is currently (as of March 2012) no adapter, nor do I expect there to be one to adapt mini-DisplayPort to Thunderbolt. (The standards were defined to be backward-compatible, not forward-compatible.)
No this will not work. Check the system requirements for the Apple Thunderbolt display.
Thunderbolt-enabled Mac computer, including MacBook Air, MacBook Pro, Mac mini, and iMac OS X v10.6.8 or later
As of yet there is no way to add a Thunderbolt port to a Mac that did not ship with Thunderbolt.