My wife was able to obtain one for us during check-in at the Encore this January. She built up some good rapport with the clerk and asked if it was possible to get a line pass. The clerk said that they normally don't have any (or didn't give them out) at the front desk but said she'd check. She then disappeared to the back for a few mins and reappeared with a line pass (this on top of the mini-upgrade to a panoramic suite). The wife responded with a nice tip of course.

I guess my point is that:
1a. If you want something in Vegas, you can't be timid to ask (standard protocol)
1b. It can't hurt being polite and friendly
2. You can obtain line passes from non-standard channels but YMMV as it's dependent on whether or not the person you're asking can and is willing to pull some strings/favours for you.