Ian -- is it possible your ISP was scanning for vulnerabilities?
Some of the automated port security probes out there will just run down a list of IP addresses and probe them for a specific port common to an attack (like the NetBIOS share folder steal).
Right now, since the US gov't squacked to MS about the Remote Procedure Call, a lot of ISPs are scrambling to get their users patched.
Of course, the RPC bug is hilarious, because everyone and their brother knew it was waiting to happen -- God, I said it the first time I heard the name!
We wait how many years before the US gov't wakes up to it . . . ?!?!
We can't stop here! This is bat country!