Thanks back!
In answer to your question, a line pass can be for a buffet, or club, or for anything you might need to wait in line for, but those two are the most common. As far as buffet line passes go, that's about the lowest level of comp you can receive, and it literally means you can bypass the wait line, which is more pertinent at some buffets than others, depending on popularity and when you're trying to eat -- if it's a busy holiday weekend at Wynn or Bellagio, for example, only valued players are likely to score a pass to skip the two-hour-or-more wait. As we would always recommend, regardless of your level of play, be sure to join the players club wherever you play or stay (these days, so many clubs award points for expenditure outside of the casino floor), and, as Jean "Queen of Comps" Scott would say, "If you don't ask, you don't get," so always ask.
Nightlife line passes are a little different, and also come in different shapes and sizes. You'll see "VIP passes" for clubs in hip bars and retail outlets around town, but what do these little business cards actually represent? On a weekend night, any Las Vegas club worth its salt is going to be packed, with an over-populated guest list, so the free "VIP line pass" someone handed you, or that you found on a website, at best will likely get you to stand in a one-hour line instead of a 3-hour line. If you're a single male or group of males, however, this might be worth it, given the clubs' bias toward female guests.
There are lots of independent promoters out there being paid to bring bodies into nightclubs and strip clubs, and some obviously have more juice than others. Twitter's a good place to find Vegas club promoters, who'll be urging you to text them for VIP access and line passes. We confess, we haven't ever tried their services, but some judicious Googling should reveal which promoters seem to be the real deal. We follow several of them with our @SinCityAdvisor Twitter, so check it out.
If you're looking at those companies that offer VIP nightlife packages and club crawls, do your research and find out which clubs you'll actually have access to, as often it's the second-tier places offering these deals. If you're not hell-bent on rubbing shoulders with the Kardashians, however, the smaller clubs can be a good bet in terms of lines, bottle-service prices, and a more laid-back vibe. There are still lots of deals to be had in this economy, so do your research before you fork over cash for something that might not be as good as it sounds.