This may be a question that has been answered before, but I need to make an ftp server accessible from the outside world from behind my Linksys WRT54G router. I know you can use the router to direct the ftp control to any machine in the private network, but isn't there a problem when the internal machine tries to set up the data link? Or, am I off in left field?

tj

Yes. If your external, public IP is 1.1.1.1 and your ftp servers local ip is 192.168.1.254 then internal users wont be able to access the FTP server by using 1.1.1.1 host by default, but they'll need to access it's internal IP.
It's because of some routing flaws.
Easiest workaround for this is if you have your own DNS server for your domain. Have all the internal clients use your DNS and create a new zone in local only view, so only internal clients will be able to resolve that domains, and those domains must resolve to local IP addresses.