Not Dark Energy
"Dark energy may not be needed to explain why the expansion of space appears to be speeding up. If our universe is like Swiss cheese on large scales – with dense regions of matter and holes with little or no matter – it could at least partly mimic the effects of dark energy, suggests a controversial new model of the universe."
As nige has noted, if the Universe is not homogeneous different regions will appear to expand at different rates. If your telescope looked in a direction of lower expansion, the Universe would appear to be accelerating. This adds to many anisotropies seen in the Cosmic Microwave Background. Though this model is very preliminary, physicists Sabino Matarrese and Rocky Kolb have published online On cosmological observables in a swiss cheese universe.
Back in March, NEW SCIENTIST published Is dark energy an illusion?
"The quickening pace of our universe's expansion may not be driven by a mysterious force called dark energy after all, but paradoxically, by the collapse of matter in small regions of space."
Just last week A Hole In the Universe indicated that the Universe is not quite homogeneous, and a cosmology including "dark energy" may be all wrong. The proposed Supernova Acceleration Probe would survey only 15 square degrees of sky before ending its service life. The whole sky has an area of 41,253 square degrees! If SNAP looked at the wrong part of an inhomogeneous sky, it could give researchers the wrong value of cosmic acceleration. Then again, disciples of "dark energy" may already have the wrong idea.
Lawrence Krauss said that supernova data "naively implied that the Universe is accelerating." The inferrence of cosmic acceleration relies on a daisy chain of assumptions, including homogeneity. It especially relies on assumption of a constant speed of light. SNAP/JDEM is subject to a review whose results will be announced Wednesday. With all the outstanding questions about its basic science, it is hard to see how SNAP can supersede projects like Constellation-X.