Comment 2 for bug 580523

This affects me as well with flashplugin-installer_10.1.85.3ubuntu0.10.04.1_amd64.deb (and all previous versions). The reason this happens is that the postinst script ignores all apt proxy settings when using wget to fetch the tarball.

I have a workaround though:
I let the download fail for exactly 20 tries -- incidentally, this can't easily be cancelled (you must 'kill' the wget process) and that is a separate but equally important bug against this package, IMO. After that, i just sudo http_proxy=http://myproxy.com:myport/ dpkg-reconfigure flashplugin-installer, or, more generally:
sudo bash -c 'eval $(apt-config shell http_proxy Acquire::http::Proxy) dpkg-reconfigure flashplugin-installer'
(replace http with https or ftp if that's what you use)

The proper way to fix this is to have the postinst script that's using wget to fetch the tarball honor what's in /etc/apt/apt.conf{,d/*}, that way the user doesn't have to set the proxy through environment variables that wget will pick up. Here's a patch that does that along with reducing the tries to a more reasonable count of 2:

--- postinst 2010-09-20 16:40:50.000000000 -0700
+++ postinst.fixed 2010-11-02 17:31:06.969230608 -0700
@@ -62,6 +62,12 @@
                echo "dirstruct = off" >> wgetrc
                echo "verbose = on" >> wgetrc
                echo "progress = dot:default" >> wgetrc
+ echo "tries = 2" >> wgetrc
+ eval export $(apt-config shell \
+ http_proxy Acquire::http::Proxy \
+ https_proxy Acquire::https::Proxy \
+ ftp_proxy Acquire::ftp::Proxy \
+ )

                # downloading the plugin
                echo "Downloading..."

I've tested this and it should be a comprehensive fix for the problem, as far as I can tell.