I did mine atop the Eiffel Tower at paris. I didn't ask Paris to make it a private deal, but they might have. You should call ahead and ask if you decide to do that there. Then we went to one of the lakeside restaurants at Bellagio to eat.
Gondala ride at the Venetian, in front of the fountains at Bellagio, private helicoptor ride or at the top of The STRAT . I proposed to my wife at Disney World's Magic Kingdom, we can never forget that day. Good Luck and make that day/night special!
HUmm.. here are a couple of ideas... contact Blue man group.. have them put it in the signage at the start of the show.. Ask Mandalay Bay to display it on there giant marquee sign out front at a certain time. Go for a helicopter ride of the strip.. do it up there. revolving resturant at the top of the strat. in front of the Fountains at the Bellagio. and umm... drunk off your ass at the craps table at the El cortez.... "Honey... we make dis point, and me and you's is getting married" then of course allow the dice to decide your fate. good luck!!
Definately take a private gondola ride at Venetian. It will cost you $50, and you have to get there in the morning to buy your ticket for the time slot you want it (day of ticket sales only), but personally I can think of NOTHING more romantic than proposing that way. If you do it, clue the Gondolier in on it so he/she is prepared and doesn't sing the whole blinking time. LOL