My answer is simple:
If it's too hot for you, it's too hot to BW.
I agree. It was 113 here today and it was too hot to even go outside and get the mail, much less babywear. Indoors I'm ok though. And indoors I shall be until about October when it dips back below 100. Bleh!
I agree with Hybrid and would rather wear DD in hot weather than not because you have a constant reading of her temp at all times. If it is too hot to wear her then she shouldn't be outside even in a covered stroller. Have you ever gone in a sealed tent in the midnoon sun? That's how I see a covered stroller.
We just got back from disney and I ended up doing a combo of babywearing in a MT and using the stroller. There were times when I was a little nervous about the heat so I would relax in a store or restaurant for a bit or inside attraction. Overall, it went really well. I also brought one of those spray fans to cool her down.