Great in theory, tough in practice. Start-ups demand specific domain expertise moreso than "general management" skills. It is very hard for an MBA to go work at a start-up directly out of school unless they have specific domain expertise in something like Healthcare or Technology.
Also, start-ups are feeling the pain of the economy as well. They are not immune from it. MBAs are a very superfluous expense for a start-up company that is looking towards an exit within the next few years - a larger corporation may be able to justify them better in a downturn with the idea of "They will feed into future senior management ranks"