Description of problem:
For the preupgrade test day, I tried to upgrade an F12 VM with only a 6GiB disk available.  The disk was nearing capacity (virt-df disk.img prior to the preupgrade attempt showed:
 fedora_12:/dev/vg_vmfedora12/lv_root   4320944    2942248    1334796   69%
Preupgrade warned during the download process that I was running low on space, and needed to clear up about 60MiB before preupgrade could complete.  I did so, by cleaning up just enough, then preupgrade finished and let me reboot.  But when running anaconda on the reboot, the system complained that / needed 700MiB more space to complete the transaction, and failed to perform the upgrade.

Version-Release number of selected component (if applicable):
preupgrade-1.1.7-1.fc12.noarch

How reproducible:
didn't try to reproduce, but probably rather easy, given enough disk space tweaking

Steps to Reproduce:
1. fill the / partition of the disk with a filler file
2. run preupgrade; when it complains about insufficient space, reduce the size of the filler file by about 1MiB more than what preupgrade complained about
3. once preupgrade is happy, proceed to reboot
  
Actual results:
anaconda complained that there was insufficient space to complete the transaction; the upgrade was aborted, and after Ctrl-C, I was rebooted into F12

Expected results:
preupgrade should be able to account for the additional disk space that will be required by the anaconda transaction size; it's not just how much space is needed to hold all the upgrade rpms, but how much those rpms will expand during installation prior to the rpm cleanups.  preupgrade should be able to give the user an accurate count of how much disk space is necessary for a completely successful upgrade.

Additional info:
I ended up using virt-resize to grow the VM disk, then after pvresize and friends, had a larger /, at which point repeating the entire exercise passed.