The cover story for the March 2002 issue of Discover magazine, "Can We Find Another Earth?" by Michael D. Lemonick, details an interesting design concept for NASA's Terrestrial Planet Finder (TPF) that is being advanced by a group from Princeton University, headed by David Spergel and Jeremy Kasdin. Under the Princeton concept, TPF could consist of *one* telescope rather than the four formation flyers NASA once envisioned. Simply stated, the Princeton concept involves the use of a "cat-eye"-shaped aperture mask (rather than multiple mirrors) to produce an Airy interference pattern that concentrates the light above and below a star, but leaves dark "wedges" to right and left where planets could be imaged.
The Princeton team estimates that this concept could "suppress the light of [a] ... star by a factor of 100,000." -- though one notes that a star at visible wavelengths typically outshines a planet by factor of 10 billion to one. However, operating at visible wavelengths allows TPF to operate much closer to Earth (at one of the Lagrangian points) rather than in the infrared at Jupiter-like distances outside the solar system dust belt. The upshot is that this concept could enable a much simpler (and much less costly) TPF to be launched.

See http://www.discover.com/mar_02/featearth.html