i am in the market for a jacket for this summer (leather or otherwise). Living in the SF Bay Area, where temperatures rarely break 90 and can be in the low 60s for much of the summer, i usually wear a pretty heavy leather jacket. Will be riding in some hot areas this summer (inland oregon, Idaho, California desert etc.) and need to rethink my gear.
Personally, I think you stay cooler in dry, really high heat when you're covered up. You basically stay cool by avoiding the sun and channeling the wind to carry away (completely) your body heat and sweat. Do it right and you'll stay bone-dry, but you're going to find that you need to drink a lot of water. Desert sucks it right out of ya.
I've worn a heavy Hein Gericke leather jacket for years, including the 7 years I lived just outside of Los Angeles. As long as the ventilation is good in a leather jacket (REALLY REALLY GOOD - just OK is not OK), they work great. If you stick with leather, you need vents on both sides of the forearms (best with gussets to create a scoop), long vents on the chest and back, and preferably under the arms too. With that, I can tell you that you can ride through the Mojave desert in mid-summer and it will be tolerably comfortable. A long-sleeve t-shirt (for the sun) and vest with chain extenders to keep the front open and catch the wind to channel it around you works well too.
Any jacket will be too hot when you're stopped, though, including mesh. You're going to cook in the desert with anything but a t-shirt. Oh, except at night, then you're going to freeze.
Get a helmet with good vents too, if you don't already have one.
The one thing I never did solve was seat heat. Some like to use those beaded covers like you see in cars.
bc