Possible linkback errors?

I was looking at my error log and noticed today I've had two very different IP addresses trying to access secure.del.icio.us/login via my site URL. They seem to be cycling through various combinations of my URL followed by that link with varying numbers of slashes and different prefixes (http, https, blank, etc.). They even try duplicating both my URL and the other one.

Since I have no connection to secure.del.icio.us other than the linkbacks I just activated a couple of days ago I have to wonder if they aren't somehow involved?

One of the IP addresses matches a user that joined but never activated, the other has no matches at all. So I'm thinking they're inbound?

vBSEO bookmark link points to del.icio.us/login , not to secure.del.icio.us/login

Ok, thanks.