Hmm. Not sure, I've never noticed this before. I'm running Outpost Firewall and occasionally see that it is blocking port scan "attacks", but haven't seen this one, nor do I get a port scan when I fire up TF and start downloading tiles.
That IP is:
Search results for: 64.105.163.108
OrgName: Covad Communications Co.
OrgID: CVAD
Address: 2510 Zanker Rd.
City: San Jose
StateProv: CA
PostalCode: 95131
Country: US
No idea who Covad is, but I think Terraserver is physically located in the bay area (can't confirm this, though), so it might be Terraserver related.
Let us know if you discover anything else, and I'll do the same.