You're not wrong in your observation that a lot of Las Vegas' more upscale eateries are open for dinner only and finding lunch options is a piece of research we've been meaning to conduct for awhile, so thanks for goading us to finally get to it.
In fact, once we looked into it, we found that more restaurants than we'd thought are open for lunch. We've stuck mainly to hotel-casino properties on the Strip, but also included a few off-Strip suggestions, like the Tillerman, which has been around for forever, but which we only just got around to trying out (we reviewed it in the 9/07 issue of the Las Vegas Advisor).
Cuisine types span everything from Continental to Mexican and sushi bars to steakhouses, and include a number of al fresco options (Olives, Maggiano's, Nine Fine Irishmen, Bouchon, Mon Ami Gabi, and Border Grill, to name a few). Since you used the phrase "upscale," we used our $$$ rating ($20+ for an entrée) as the minimum-price criterion. Some of the restaurants listed are on the casual side (Todai, Sushi Fever, and rumjungle, which serves a burger and nachos-type menu noon-4 pm), while others are decidedly fancier. We'd recommend the Palm (at the Forum Shops at Caesars) or the Tillerman for a business lunch, Rosemary's or Eiffel Tower for a romantic treat, and Capital Grill or Top of the World for a view.
For our full findings, click this link.