What happens if one of the source.list are no longer found.?

[Ramasubramanian Ramesh <rramesh@ti.com>]

Hi,

  I use testing as my standard distribution. 

I got xserver-xfree86 4.3 from backports some time back (stable/pre/xfree86) and this directory is
no longer available in backports. I suspect this leaves many dependencies hung in apt. It does not
want to upgrade xterm xbase-clients etc and many others (about 50 or so packages). 

Now I see 4.3 is in unstable (and not in testing). I think that getting xfree86 4.3 from unstable
will get me going (I am not an expert in apt-get so please take it with a grain of salt when I say
"I think") But I do not want to turn my entire distribution to unstable. Any suggestions?
 
Should I wait (as long as it takes) without upgrading these (dependency broken) packages until
unstable becomes testing? Or should I do something else?
BTW, I can provide the apt-get -u -upgrade output if that is going to help in any way (it is quite
long with debug option turned on)

Thanks for your help.
Ramesh