:~/gnash-0.7.2$ sudo apt-get -t etch-backports install mozilla-plugin-gnash
Reading package lists... Done
Building dependency tree... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.

Since you only requested a single operation it is extremely likely that
the package is simply not installable and a bug report against
that package should be filed.
The following information may help to resolve the situation:

The following packages have unmet dependencies.
  mozilla-plugin-gnash: Depends: gnash (= 0.7.2-1lenny1~bpo40+1) but it is not going to be installed
E: Broken packages
:~/gnash-0.7.2$ sudo apt-get -t etch-backports install gnash
Reading package lists... Done
Building dependency tree... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.

Since you only requested a single operation it is extremely likely that
the package is simply not installable and a bug report against
that package should be filed.
The following information may help to resolve the situation:

The following packages have unmet dependencies.
  gnash: Depends: libgnash0 (= 0.7.2-1lenny1~bpo40+1) but it is not installable
E: Broken packages
:~/gnash-0.7.2$ sudo apt-get -t etch-backports install libgnash0
Reading package lists... Done
Building dependency tree... Done
Package libgnash0 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package libgnash0 has no installation candidate