Finder says I have 20GB free. Why does Bootcamp say I don't have 10GB free?
You may be just reading the wrong figure in Disk Utility, have a look at the quote below from the apple discussion forums


  Disk Utility will show two different
  "free" readings, depending on where
  you look.
  
  Sounds odd, but they're for different
  purposes. The larger figure, which the
  Finder and other things will use, is
  the total free space available.
  
  The second is the amount of free space
  available for a new partition. It's
  shown when you select the top line for
  the drive, click the Partition tab,
  then select a partition, the
  "Available" amount may be smaller. 
  
  This is because partitions, unlike
  folders, must occupy contiguous space
  on a disk. The data on your OSX
  partition is scattered over it, not
  all nicely packed together. That's
  normal, as you add, update, and delete
  files, not every nook and cranny is
  used again immediately.
  
  But whether there's 7 GB free or 18,
  that probably isn't enough to be
  adding a partition; it's probably too
  little for OSX to operate well.
  There's no "hard and fast" rule, but
  you should always have at least 10 GB
  or 10% of your OSX drive free; many
  folks say 15% or more.
  
  You could back up your drive (on two
  different HDs, to be safe), reformat
  it to 2 parttions, then restore the
  backup. But it's going to be very
  tight, and you'll likely have
  performance problems because of it.
  
  Try to delete anything you don't need,
  and/or offload stuff you don't use
  often and delete it.