I get quite a lot of those as well. As long as they're blocked, I don't really worry... It means the router is doing its job. It always amazes me how frequent these are.
Maybe you can check if there are any repeating offenders, and block the IPs altogether.
If you're concerned, you can make sure you have iptables setup to block everything you don't need in your computers, and/or monitor your incoming connections.
Firestarter can also show what connections it blocks when it's running in the tray. I personally like to use conky to display outgoing and incoming connections on the root window, so I can see what's going on in real time.