Houston, we still have a problem. To my knowledge and belief, I have carefully followed the Official Fix instructions given in bug #173890, including apt-get clean and apt-get autoclean.

On my Gutsy system:

I have only Ubuntu repositories enabled (comment lines snipped):
  root@odyssius:/home/simon# cat /etc/apt/sources.list
  deb-src http://archive.ubuntu.com/ubuntu/ gutsy restricted main #Added by software-properties

  deb http://gb.archive.ubuntu.com/ubuntu/ gutsy main restricted
  deb-src http://gb.archive.ubuntu.com/ubuntu/ gutsy restricted main multiverse universe #Added by software-properties
  deb http://gb.archive.ubuntu.com/ubuntu/ gutsy-updates main restricted
  deb-src http://gb.archive.ubuntu.com/ubuntu/ gutsy-updates restricted main multiverse universe #Added by software-properties

  deb http://gb.archive.ubuntu.com/ubuntu/ gutsy universe
  deb http://gb.archive.ubuntu.com/ubuntu/ gutsy-updates universe
  deb http://gb.archive.ubuntu.com/ubuntu/ gutsy multiverse
  deb http://gb.archive.ubuntu.com/ubuntu/ gutsy-updates multiverse

  deb http://security.ubuntu.com/ubuntu gutsy-security main restricted
  deb-src http://security.ubuntu.com/ubuntu gutsy-security restricted main multiverse universe #Added by software-properties
  deb http://security.ubuntu.com/ubuntu gutsy-security universe
  deb http://security.ubuntu.com/ubuntu gutsy-security multiverse
  deb http://gb.archive.ubuntu.com/ubuntu/ gutsy-proposed restricted main multiverse universe
  deb http://gb.archive.ubuntu.com/ubuntu/ gutsy-backports restricted main multiverse universe
  root@odyssius:/home/simon# 

I then purge and attempt to reinstall the package:
  root@odyssius:/home/simon# apt-get remove -y --purge flashplugin-nonfree
  Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  The following packages will be REMOVED
    flashplugin-nonfree*
  0 upgraded, 0 newly installed, 1 to remove and 6 not upgraded.
  Need to get 0B of archives.
  After unpacking 160kB disk space will be freed.
  (Reading database ... 206773 files and directories currently installed.)
  Removing flashplugin-nonfree ...
  Purging configuration files for flashplugin-nonfree ...
  root@odyssius:/home/simon# apt-get install flashplugin-nonfree
  Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  Suggested packages:
  konqueror-nsplugins ttf-xfree86-nonfree xfs
  The following NEW packages will be installed
    flashplugin-nonfree
  0 upgraded, 1 newly installed, 0 to remove and 6 not upgraded.
  Need to get 0B/18.2kB of archives.
  After unpacking 160kB of additional disk space will be used.
  Preconfiguring packages ...
  Selecting previously deselected package flashplugin-nonfree.
  (Reading database ... 206767 files and directories currently installed.)
  Unpacking flashplugin-nonfree (from .../flashplugin-nonfree_9.0.48.0.2+really0ubuntu12.2_i386.deb) ...
  Setting up flashplugin-nonfree (9.0.48.0.2+really0ubuntu12.2) ...
  Downloading...
(snipped dl stuff)
  --17:17:03--  http://fpdownload.macromedia.com/get/flashplayer/current/install_flash_player_9_linux.tar.gz
           => `./install_flash_player_9_linux.tar.gz'
  Resolving fpdownload.macromedia.com... 88.221.178.70
  Connecting to fpdownload.macromedia.com|88.221.178.70|:80... connected.
  HTTP request sent, awaiting response... 200 OK
  Length: 3,044,538 (2.9M) [application/x-gzip]
    0K .......... .......... .......... .......... ..........  1%  333.92 KB/s
(snipped dl stuff)
  17:17:25 (135.73 KB/s) - `./install_flash_player_9_linux.tar.gz' saved [3044538/3044538]

  Download done.
  md5sum mismatch install_flash_player_9_linux.tar.gz
  The Flash plugin is NOT installed.

The package policy is:
  root@odyssius:/home/simon# apt-cache policy flashplugin-nonfree
  flashplugin-nonfree:
    Installed: 9.0.48.0.2+really0ubuntu12.2
    Candidate: 9.0.48.0.2+really0ubuntu12.2
    Version table:
   *** 9.0.48.0.2+really0ubuntu12.2 0
        500 http://gb.archive.ubuntu.com gutsy-updates/multiverse Packages
        100 /var/lib/dpkg/status
       9.0.48.0.2+really0ubuntu12 0
        500 http://gb.archive.ubuntu.com gutsy/multiverse Packages
  root@odyssius:/home/simon#

One suspects that Adobe have once again updated the package at the URL which the downloader attempts to fetch.