I think Locke was lying when he said he took Christian's form.
1. Why would he lead Candidates to water and help Jacob by keeping them alive.
2. Christian appeared off island on the frieghter and in Jack's waiting room in LA which the Smoke monster obviously can't do.
3. In that early episode Christian lead Jack to the caves and then dissappeared. This goes against Smokey's MO as seen in the Richard episode and with other characters. Why not talk to Jack and say "hey this weird island has a big bad mean old man keeping you from being rescued." At that point in time Jack had just been there a few days and would be easily manipulated.
I don't think it's an oversight in the writing, more that he was lying to Jack to try and win him over.
