The availability of a commercial transport to orbit as envisioned in the President’s proposal cannot be predicted with any certainty, but is likely to take substantially longer and be more expensive than we would hope.The whole letter is here. This isn't the first time prominent moon-walkers have publicly split with the current administration. Armstrong's fellow Apollo 11 crew member Buzz Aldrin recently opined that Obama was not doing enough to further a manned mission to Mars.
For The United States, the leading space faring nation for nearly half a century, to be without carriage to low Earth orbit and with no human exploration capability to go beyond Earth orbit for an indeterminate time into the future, destines our nation to become one of second or even third rate stature. While the President's plan envisages humans traveling away from Earth and perhaps toward Mars at some time in the future, the lack of developed rockets and spacecraft will assure that ability will not be available for many years.
Without the skill and experience that actual spacecraft operation provides, the USA is far too likely to be on a long downhill slide to mediocrity. America must decide if it wishes to remain a leader in space. If it does, we should institute a program which will give us the very best chance of achieving that goal.
Update: Aldrin supports the president on his recent plan. "Today I wish to endorse strongly the President’s new direction for NASA," he writes.