Sparse checkouts

Hi,
I'm working with a repository with a very large number of files and Git is 
appealing because it officially supports sparse checkouts unlike the other 
DVCS tools. However, all of the usage examples that I've come across have 
you checkout the full repository and then prune the undesired files as such:

  git clone <dir>
  git config core.sparsecheckout true
  echo "<dir>/" > .git/info/sparse-checkout
  git read-tree -m -u HEAD

I tried adding "-n" to the clone command but then none of the missing 
directories are checked out by read-tree. I can manually check them out 
and everything seems to work fine but I can also manually check out a bunch 
of other directories and read-tree, reset, etc all seem to ignore the extra 
directory even though they aren't specified in the sparse-checkout file. Is 
this use mode just not well supported by git yet or am I missing something? 
I'm new to Git so I don't expect to be able to figure it out easily and I 
haven't been able to find the answer elsewhere.

Thanks,
Uri