I think this sets up less of a precedent, on the writers' behalf, for just what's going to happen. Before, it was like you said, Arthur would've likely hurt Merlin for the magic, but now, it's all becoming just a little fuzzier. It's even less predictable, which works in the writers' favor.

And yeah, the Gaius/Uther in this episode was wonderful. Makes my slashy fangirl heart go boom boom boom. ^_^ But on a slightly more serious note, it is kind of sad, and something we can all hope never happens - yet, when it's clear just how close these two are, it's also all too easy to see the boys headed in the same direction. :( *pouts*