I am leaving for Vegas in a couple of weeks and have been doing some research into happy hours also. There are a couple of posts if you do a search for las vegas happy hours on chowhound, like this one: http://chowhound.chow.com/topics/782841.
This post also has a link for a great website that has a listing of the happy hours on the strip. Some of them have the menu listed or give you an idea of what kind of prices they offer for their happy hours. Also, if you go to the restaurant/lounge websites, they sometimes have information about their happy hours.
Ones that stand out for me is Seablue in the MGM, 5:30 - 7 pm except on Saturdays for their $1 oysters and other tasty seafood appies, $3 Stellas and $5 Sangria and daily specialty cocktail., Diego, also in MGM from 4 - 5:30 for margaritas and some mexican. Boa Steakhouse from 4 -7 pm, located in the Forum Shops at Caesar's Palace for $5 appy and drink specials. Lots of good happy hours to be had for sure.....I've started a spreadsheet so that I know where to go depending what vicinity we are in around happy hour.