Sayid tells Locke he'll "have to do better than that" regarding his explanation for the explosives. Apparently not, because soon enough, Locke's along for the jaunt, with a lot of unanswered questions. Why bring along a potential traitor without taking the time to have things out first? Now that seemed like an appropriate time for a quality interrogation. "We'll talk about this later"??? I'd say the writers "have to do better than that."

Other than that, it was great to see Sayid back again, especially with his comment about the infallible magic markings.

I don't think Sayid completely trusts Locke, but I also think that Sayid doesn't really see Locke as a potential traitor, either. I think Sayid took more issue with the fact that Locke hadn't revealed that he knew about the plastic explosives, which was very important, and something Sayid should have known about.
Perhaps the problem lies in the fact that each of them sees himself as the leader of the expedition. Sayid is looking for a little teamwork, and Locke just isn't very good at sharing.
I think Locke may also get on Sayid's nerves just because he's so out there and Sayid is so grounded in the real world.
However, I think that Sayid is pretty aware that Locke is extremely stubborn, and that he is not going to be able to get anything out of Locke that Locke isn't willing to share (at least, not without resorting to completely unjustified means). If Sayid had really felt threatened by Locke, I'm sure there would have been much more of an issue than there was.
I do agree that the writers could have wrapped that up a little bit better, but they do have the tendency to go for every bit of mystery they can get, unfortunately. At this point, I'll take what I can get--these past few episodes are such a dramatic improvement from the beginning of this season that I am not feeling particularly picky. I'm just hoping this won't come back to bite Sayid later.

Yes, I agree, the last two episodes were a dramatic improvement.