iPhoto not recognizing memory card, but mounts in Finder
Connect the memory card and launch the program Image Capture (found in the Utilities folder in both /Applications/ or in Launchpad, if you're running Lion. In the lower left corner of the screen, there should be a menu that says Connecting this camera opens...; from the menu, select iPhoto from the list.

This should make iPhoto launch when you connect this memory card.