the ZIF is basically just a flat flex cable of the proper pitch and number of conductors. Digi-Key sells a wide variety. Also the connectors for said cable.
It should be a relatively simple matter to match up the pinout of 2.5" IDE with the 1.8" ZIF IDE and build a tiny circuit board to translate between connectors. Once that's done, slide a 1.8" ZIF cable into the connector on your adapter and slide the other end into somethign like this:
Toshiba MK2431GAH 240GB iPod 1.8" HD
Although lower capacity versions are available for considerably less money. You would think that the 1.8" drive would leave room in the 2.5" bay for the cable adapter.
This is the type of hard drive that the Dell Latitude D420 and D430 use, which incidentally, make excellent Hackintosh laptops. I just finished installing Snow Leopard on a D430 and am in slow motion testing. Looks good so far. The D430 can commonly be had for about $50, but as you can see, a 240GB hard drive cost almost twice that amount.