A standard Debian install of apache will have the following fragment of configuration:
Listen 80
<IfModule mod_ssl.c>
# SSL name based virtual hosts are not yet supported, therefore no
# NameVirtualHost statement here
Listen 443
</IfModule>
This is telling apache to listen on port 80 and to listen to port 443 if mod_ssl is configured. In your case you'd want:
Listen 80
Listen 8080
You need to make sure you run a restart, not a reload operation on apache for it to pay any attention to new Listen directives. The safest thing to do is to stop apache, make sure it is dead and start it again.
If this configuration does not work, check the log files for any error messages. You could use "netstat -lep --tcp" to see if there is anything listening on port 8080. Finally, if everything else doesn't work, try running apache under strace to see if it's trying to bind to that port and failing, but not logging the problem.