Knowing now what we know about John Locke and his past and early time on the island we understand that he was paralyzed from the waist down from the time he was pushed out the window and touched by Jacob to his survival on 815 Locke has always been connected to the island.
I noticed that the episode Tabula Rasa (Kate centric) is also the very first episode that Locke was really shown to be a known character. I know I'm reading a little to much into the whole Blank Slate thing, but I find it interesting that with Nemesis taking Locke's form that maybe Nemesis may have noticed Locke around this time as someone who is more special than anyone else on the island.
Maybe Nemesis found his own Blank Slate just as Kate found hers during this episode. Like I said OBVIOUSLY I'm reading to much into it, but it was just an idea.