In reply to Smelly Fox:
Climb in Red Rocks in August and you'll confirm the old adage about mad dogs and Englishmen. It is brutally hot---you'll earn your moniker. Quite a few of the routes in the canyons are in the shade, but you still have to walk there and back in full sun in 104 degree (F) temperatures. Start at or before dawn and if you're done before sunset, you might think about a siesta before hiking out. Fail to carry enough water and you could be in serious trouble.
I think all the locals head up to Mount Charleston in the summer. It's 45 minutes North of Vegas and 6,000 feet higher; it isn't the desert and temps are bearable (20 degrees coooler than Vegas during the day).
Limestone sport climbing, starting at 5.10. Have a look at
http://www.mountainproject.com/v/mt-charleston/105850026 . The guidebook is called Islands in the Sky, by Randy Leavitt, Dan McQuad and Mick Ryan. You can pick it up at the local shop, Desert Rock Sports
http://desertrocksportslv.com/index.html .
But really. Stick with the Sierra high country. You've got everything from alpine scrambles to towering granite walls with 5.12 routes on them, and some epic peak traverses at various levels of difficulty. It's nuts to abandon that for the desert oven of Vegas in August.