Hypersmurf
Moderatarrrrh...
So, in Hypersmurf's example, you are using the Nimble Attack power at all points during that, as you are still using the 'Use a Power' action. However, if said power includes shifts and such, those shifts and such -explicitly- deny the interruption via opportunity actions during those shifts.
Let's say we have a power that lets me make a ranged attack, move two squares, and make another ranged attack. (I'm sure there probably is one, but I can't be bothered hunting for it right now

And let's say I'm within the threatening reach of a Boneclaw, with its Relentless Opportunist ability allowing it to make multiple OAs in a single turn.
The first attack provokes, and leaving the first square provokes, and leaving the second square provokes, and the second ranged attack provokes, right?
Now let's say the power swaps the move for a 2-square shift.
The first attack provokes, and leaving the first square doesn't provoke, and leaving the second square doesn't provoke, and the second ranged attack provokes, right?
So even though shifts don't provoke OAs, I still provoked OAs with ranged attacks as part of that power.
Now, in the Nimble Attack example, I use a ranged power in a square adjacent to an enemy, and I shift out of that square, and I make a ranged attack. The shift does not provoke. But using a ranged power in a square adjacent to an enemy does. And if I'm threatened by the time I make the ranged attack, making a ranged attack does as well.
Even though the shift doesn't provoke, that doesn't change the other ways in which Nimble Attack provokes OAs. And one of the ways Nimble Attack can provoke an OA, since it's a ranged power, is if it is used in a square adjacent to an enemy.
-Hyp.