I think this was hand lettered.

David, there is a fair bit written about this sign already on Typophile.

You'll find a pretty good insight in this thread.
http://typophile.com/node/7785

There' also a post about half way down this thread by Stuart Sandler, who seems to know a lot about Las Vegas signage.
http://typophile.com/node/5909

I also pulled out this link, which has an interview with Betty Willis, the original designer of the sign:
Betty Willis interview