So I have some radially laced MTB wheels and I went to fiddle with it with my crappy spoke wrench, duh (new one on the way) and the aluminum nipple flats rounded off and I can see a crack where it meets the fatter part of the nipple.

So am I looking at a $30 visit to the LBS for detension/replace?

I have an assortment of nipples here from a wheel set I broke down. Could I remove and replace just this one with a brass nipple and bring it back to true?

It seems like the flats were not a threaded portion of the nipple so I wonder if it is still just as strong and can be ridden on for the rest of the week?

Just replace it with brass. Al nipples are ******** unless you're on top of maint.

Just swaparoo or should i take it to the LBS and have them take it all down and retension?

If that wheel is going to fail, be it the spokes or nipples it's going to fail regardless of whether you detension that wheel to replace that nipple or not.

Hmmm, not sure if that's a concern or not. I rode in on it today and it's still fine. I'll swap it out for brass when I have time and see how it all goes.

Yes.

You're worrying about nothing. Just swap it for a brass one and tension it up and check the area for true as you do. No need to de- and retension the wheel.

However if you want something to worry about I'd be worrying about riding when it's quite possible that the spoke is not holding much of anything. Riding with a potentailly loose spoke isn't something you want to do for more than you absolutely have to.