If you were to drop me on the Strip and tell me to go find the nearest ham and cheese croissant, I'd make a beeline for the Paris hotel. I'm positive that Lenôtre inside the Paris has them. I didn't see them with my own eyes when I went last, but I wasn't really looking for them, and they have darn near every other kind of French pastry I could imagine, so a ham and cheese croissant is hardly a stretch. Bouchon Bakery at the Venetian is a pretty solid bet, and from what I've heard, if they have them they'll be way better than the ones at Lenôtre. I was a bit dismayed with the pastry I had from Lenôtre; while the pain au chocolat was utterly delicious, I expect croissant pastries to practically explode in a shower of flaky crumbs, and it held very firmly together. Good news for business-suited executives going to conferences down the hall, not such good news for Chowhounds.
There's also Jean Philippe Patisserie inside the Bellagio, which I can't vouch for either way but would be willing to take a guess that they would have them there.
re: JK Grence the Cosmic Jester
I have just returned from a week in Vegas....and yes Bouchon does have savory Croissants... Ham and Cheese included!
One morning I grabbed a Ham and cheese baguette and shared it w/ my Boss...we were in a rush to get to a conference! It was fabulous ( I could have devoured the entire sandwich!). Bouchon doesn't disappoint~