Description of problem:
I tried to do preupgrade from F13 to F15, selecting alpha releases. Prior to this I did 
yum update preupgrade --enablerepo=updates-testing
rpm -e old kernels to free space in /boot

Version-Release number of selected component (if applicable):
preupgrade-1.1.8-1.fc13.noarch


How reproducible:
100% (3 of 3 attempts)

Steps to Reproduce:
1.use F13 preupgrade
2.wait for request to reboot
3.reboot and note that grub is unchanged
  
Actual results:
only the old release is offered no upgrade happens

Expected results:
upgrade starts

Additional info:
Console output suggests a file to be downloaded is not found where expected. It looks like this error is not trapped and the GUI continues as if upgrade is possible:

[root@pricey ~]# preupgrade
Loaded plugins: blacklist, whiteout
No plugin match for: rpm-warm-cache
No plugin match for: remove-with-leaves
No plugin match for: auto-update-debuginfo
preupgrade-main (mirrorlist) 
  url: http://mirrors.fedoraproject.org/mirrorlist?repo=fedora-15&arch=$basearch
  now: http://mirrors.fedoraproject.org/mirrorlist?repo=fedora-15&arch=i386
preupgrade (mirrorlist) 
  url: http://mirrors.fedoraproject.org/mirrorlist?path=pub/fedora/linux/development/15/$basearch/os/
  now: http://mirrors.fedoraproject.org/mirrorlist?path=pub/fedora/linux/development/15/i386/os/
unknown metadata being downloaded: repomdTroRrBtmp.xml
preupgrade-rpmfusion-free-rawhide (mirrorlist) 
  url: http://mirrors.rpmfusion.org/mirrorlist?repo=free-fedora-rawhide&arch=i386
  now: http://mirrors.rpmfusion.org/mirrorlist?repo=free-fedora-rawhide&arch=i386
unknown metadata being downloaded: repomdNXizBdtmp.xml
preupgrade-rpmfusion-nonfree-rawhide (mirrorlist) 
  url: http://mirrors.rpmfusion.org/mirrorlist?repo=nonfree-fedora-rawhide&arch=i386
  now: http://mirrors.rpmfusion.org/mirrorlist?repo=nonfree-fedora-rawhide&arch=i386
unknown metadata being downloaded: repomdG_Kq8otmp.xml
unknown metadata being downloaded: MEMORY
Fetched treeinfo from http://mirror.netrino.co.uk/mirror.fedora.com/development/15/i386/os//.treeinfo
treeinfo timestamp: Fri Apr 15 23:49:02 2011
/boot/upgrade/vmlinuz checksum OK
/boot/upgrade/initrd.img checksum OK
Traceback (most recent call last):
  File "/usr/share/preupgrade/preupgrade-gtk.py", line 259, in on_assistant_apply
    self._do_main()
  File "/usr/share/preupgrade/preupgrade-gtk.py", line 278, in _do_main
    self.main_preupgrade()
  File "/usr/share/preupgrade/preupgrade-gtk.py", line 500, in main_preupgrade
    stage2file = self.pu.retrieve_non_critical_files()
  File "/usr/lib/python2.6/site-packages/preupgrade/__init__.py", line 571, in retrieve_non_critical_files
    self._retrieve_file(self.mainimage, targetdir, reserve_space=extra_space)
  File "/usr/lib/python2.6/site-packages/preupgrade/__init__.py", line 451, in _retrieve_file
    tmp = self.instgrab.urlopen(fileinfo)
  File "/usr/lib/python2.6/site-packages/urlgrabber/mirror.py", line 413, in urlopen
    return self._mirror_try(func, url, kw)
  File "/usr/lib/python2.6/site-packages/urlgrabber/mirror.py", line 386, in _mirror_try
    mirrorchoice = self._get_mirror(gr)
  File "/usr/lib/python2.6/site-packages/urlgrabber/mirror.py", line 283, in _get_mirror
    raise URLGrabError(256, _('No more mirrors to try.'))
urlgrabber.grabber.URLGrabError: [Errno 256] No more mirrors to try.