hey mate, leopard's firewall comes default as 'off'. In other words it will allow all incoming connections. If you want to check, in system preference click security, then choose the firewall tab. Make sure it's set to 'allow all incoming connections' just incase.
My question is, are you connected to a router/modem to access the internet? either with airport or ethernet? If so what you might need to do is go into the setup for the router (usually you type in an address into safari or firefox, type in a password if necessary and it'll bring up your router/modem, eg, 192.168.0..... the address would be with the manual of ur router/modem).
From there you will need to look for either a tab called 'port fowarding' or 'firewall' or 'filter' or something like that. Then you need to 'allow' access through the port number, in your case '9090'.
Once you think that you've set it up to be open, restart transmission.
If this is the solution it is possible the modem/router could have been reset or something... i have always used transmission and when i first upgraded to leopard i think it was having a number of issues. I've since done a clean install of leopard and transmission 0.92 onwards (and indeed all my apps) is working great. hope i helped