This turned out to be a very timely question; we'd already penned the answer when the announcement came on Friday of Harrah's new group-wide "Buffet of Buffets" deal, which we reported in todays news, so here's an updated version to this answer.
The first buffet to build on the "all you can eat" concept with the "all-you-can-eat-all-day" deal last year was Excalibur, where $25 got you a wristband that entitled you to each breakfast, lunch, and dinner (a dubious opportunity at what consistenly ranks as one of the worst-rated buffets by LVA readers). This deal is still in effect, but the price has since gone up to $29.99 (or $39.99 if you'd like to add a lobster to your order).
Sister MGM Mirage properties then followed suit, with MGM Grand introducing an all-day pass for $29.99 (now discontinued) and then Luxor (also $29.99, or $34.99 on the weekend). This latter offer is still available, as is the Stratosphere's $29 all-day pass (same price, but you must have a players card).
Next Palace Station got in on the act with an all-day pass ($19.99), which is still in effect, as is the Orleans' (Sat-Thurs, $21; Fri, $25). Circus Circus ($25), Imperial Palace ($25), and the Rio ($24) all have an all-day price, but Mandalay Bay had discontinued its day pass.
While, as we wrote in a previous Spotlight on... feature, there are some obvious drawbacks to this concept, including the fact that some buffets don't change their offerings that much, especially between lunch and dinner, and some of these buffets are distinctly better than others. One of the great things about a trip to Las Vegas is being able to experience so many different things, so you might well not want to stay confined to one eatery. On the other hand, as a one-off bargain play, for example, rather than something you take advantage of every day during your stay, these offers represent sizeable savings. If you plan a day of activities around your hotel, then the bargain buffet play could be really convenient, as well as being a money-saver.
Fast forward a few days since this answer was first written and, as referenced above, Harrah's last Friday announced an innovative new promotion beginning April 12 that encompasses all seven of their Las Vegas properties that have buffets. Now, for $29.99 you can buy a "Buffet of Buffets" pass that's valid for a full 24 hours from the time of purchase and entitles you to eat at the buffets at Caesars, Flamingo, Harrah's, Imperial Palace, Planet Hollywood, and the Rio (Carnival World) for the duration of the pass. As our contact at Planet Hollywood explained, that's better than the other all-day passes around town that are limited to one property -- if you time it right, you could fit in four meals at four different buffets, if you wanted to, and with dinner alone at Paris running at $24.99, or $27.99 at Planet Ho's Spice Market, that's a pretty awesome deal.
For all the times and prices at these (and all the other) buffets in town, check out our Buffet listings, which are updated every month and where you can also find details of specialty nights, plus other readers' ratings and comments.