Is there enough land to grow cellulose ethanol needed to supply the
world's fleet of automobiles in the future?

No, not even close.

Assuming markvmd is corect, we can produce perhaps 1% of the fuel for
automobiles by agriculture, before we upset world food production
seriously, and seriously deforest our planet to produce ethanol. Even
1/10 th percent may be useful as every little bit helps. We need to
persue hundreds of alternative energy ideas as it is likely none will
provide more than a few percent of the world's energy needs. Ethanol
is typically made from sugar. How do you convert cellulose to sugar?  
Neil

Cellulose ethanol -  Cellulose is a sugar.  It is one your stomach can
not digest. Cows use their four stomachs to break it down using
bacteria.

Mollasses mills break down some of the cellulose in sugar cane using a
common acid or base (forget which)  The brewers can make more rum that
way.

Brazil has a major fleet based on ethanol.  They did this by the
simple step of making it very hard to import oil.  Ethier banned or
massive tax (again i forget.)

If you wait around long enough cellulose turns into peat and then
coal.  Takes a while though.

It doesn't really matter if there's enough land to produce enough
ethanol-- ethanol actually uses MORE fossil fuels to produce it than
would be used if automobiles were powered by those fossil fuels
themselves.

I wish it were any other way, believe me; I am even from Illinois,
which is huge on corn and very pro-ethanol. If ethanol were the
solution, Illinois would become much richer than it is and I would
love that!

But ethanol is very inefficient to produce; it uses tons of fossil
fuels and the amount of corn that is needed just to produce a teeny
bit is huge.  It also produces more emissions than regular fuels,
which is a huge problem as far as global warming goes!

Basically, instead of putting our money and time into ethanol, we need
to produce hydrogen fuels or something like that.

You should look at cellulosic ethanol and forget about corn.  Its true
that the ratio of energy in/energy out isn't good for corn based
ethanol.  But the energy potential of cellulose is higher than that of
the starches which corn produces.  There are already some good
prospects for crops that have very high cellulose content and they
haven't even started to optimize them yet.  The original food output
of modern corns ancestor wouldn't be worth farming.  Also, the plants
that could be grown just for cellulose production do well in poor soil
that wouldn't support food crops anyway.  Things like switchgrass are
perrenials so they won't even need reseeding.  And they don't require
the same investment either.  No real need for fertilizer, soil
preparation etc... means that it actually takes less energy to grow
and stores more energy than corn.  So, to sum it up the answer is no
there probably isn't enough land based on current technology but given
the advances just in the last couple of years I don't think its
unreasonable to think we could push ethanol up over the halfway mark
for auto fuel.