There's been a lot written (including material in this blog) about the known and theorized problems with the extremely elongated Ares I/Orion stack NASA plans to use in returning astronauts to orbit after the Space Shuttle is retired. This article describes the biggest problem, oscillation that could cause the stack to shake severely in the first minutes of flight. NASA does not expect this to delay the Constellation plan to return humans to the Moon by 2020, but it's increasingly hard to believe that problems with the rocket originally sold as a "safe, simple, soon" solution will not damage the schedule or cause potentially crippling cost overruns.
Also see www.nasawatch.com, where Keith Cowing posted some detailed questions to NASA about Ares/Orion problems and got new answers, some of them clear and to the point and some very fuzzy. Cowing reports it's increasingly clear from internal documents that the first Ares flights will be delayed, even though NASA has denied that looking at an extended schedule was anything but a contingency exercise.
COMMENT: I support the Vision for Space Exploration and believe NASA can get the job done. But the agency seems to be going out of its way to insist all Area/Orion problems can be solved without major budget or schedule impacts. As a student of history, specifically the history of space programs, I've stopped believing them. NASA should be much fore forthright and forthcoming.
USUAL REMINDER: These are my personal opinions as a freelance science writer, nothing more.