I installed 'Leopard' also and had no problems with 'Limewire' connecting.
You stated that you 'turned off' your 'Firewall' (under 'System Preferences > Security')?
Did you set the 'Firewall' to 'allow all incoming connections'?
Is your Internet Service Provider blocking you?
Some ISPs have started this practice to prevent users from 'hogging' all the bandwidth (e.g.: Comcast).
Also, if you have not done so already, try locating the 'Limewire' preference file in the home account 'Library' folder and delete it. Then re-install the latest version of 'Limewire'. You are using the 'Pro' version, right?