You can change the site that comes up on port 80. You're off to a good start by creating a new site at port 8080.
1. Check /etc/apache2/sites/virtual_host_global.conf to make sure that your server is listening on port 8080:
Listen *:80
Listen *:443
Listen *:8080
The "8080" line should have been added when you used Server.App to add a new site at port 8080.
2. Edit /etc/apache2/sites/0000_any_80_.conf and change the VirtualHost entry from "*:80" to "*:8080". Save.
3. In the same folder, edit the .conf file for your newly-created site (the one at port 8080). Change its VirtualHost entry from "*:8080" to "*:80". Save.
4. In Terminal, type the command
sudo apachectl restart
5. Now if you browse to http://localhost, you should see your new site. If you go to http://localhost:8080, you'll get Lion's collaboration portal, the old default.
Exit and reopen Server.App. You should now see port 8080 (grayed out) shown next to the default Web site.
This isn't perfect. On my machine, this maneuver causes the default collaboration site (now localhost:8080) to lose track of its landing page icons, and I expect its confusion runs deeper than that. I don't use those services, so I haven't tried to fix it.
Best of luck, Mike.