It pays to be flexible. NASA has quietly begun studies for ARES IV, which will combine 5-segment solid rocket boosters with the upper stage being developed for ARES I. The full stack is 113 m tall, taller than the 1960's Saturn V. This offers the promise of boosting a Lunar Lander to the Moon in one launch, without waiting for the CEV. The CEV would launch on a separate ARES IV and rendezvous in lunar orbit.
This also may be cheaper to develop than the ARES V. There is a risk that after ARES I is developed, funding will run out for ARES V and we will be stuck in low Earth orbit. NASA has budget issues, and is looking for something to cut. The Vision is top priority, so something else would have to go. NASA Chief Michael Griffin talked about this to AVIATION WEEK:
"I will do everything I can to keep Orion and Ares I on schedule," he says. "That will be right behind keeping shuttle and station on track, and then after that we'll fill up the bucket with our other priorities.
"The ideal candidate is a fairly new, lower priority effort where not a lot of money has already been invested, and by stopping it now you can react and not have to spend future money that you know you're not going to get," he says. "If we don't find the ideal candidate we'll look for less ideal candidates."
That sounds a lot like the Beyond Einstein Programme. The Joint Dark Energy Mission in particular has been getting by on funding from DOE, and is not considered a high priority. After years of work, they can not even agree on a concept. If funding for JDEM gets cut, supernova researchers have themselves to blame. UPDATE: DOE also faces a funding shortfall; what will they cut?