Note to self…don’t mess with apps built on the premise “sex sells.”

Pepsi learned this lesson the hard way recently after launching a controversial iPhone/iPod application called “AMP UP Before You Score.” Here’s some Wall Street Journal coverage.

The goal was to get guys to buy PepsiCo’s Amp Energy drink rather than industry leader Red Bull, but the language left a bit to be desired. The free app was advertised as a “road map to success with yours favorite kinds of women,” and delivered pick up line advice for various stereotypical archetypes, including “nerd.”

Unfortunately for Pepsi, users spoke back, reviewing the app as “sexist and stupid.” Even joining related conversation on Twitter to apologize didn’t help users amp-down, since Pepsi kept the app active while it evaluated options.

What’s systemically interesting about this app release isn’t the app itself, but rather the fact that PepsiCo responded by listening to vocal users and pulling something about which people were displeased.

The mobile market + realtime social network responses = new era of consumer response? Probably so. No one wants to make headlines for ignoring customers, especially not those on Twitter or Facebook.

This isn’t the first time users have vocalized displeasure over advertising they perceive as insulting using social networks – remember the hullabaloo over Motrin Moms?

Sometimes, however, we just want something slightly naughty – in other words, we should be able to choose our own mobile content without censorship. Exhibit A = the iPhone app and website combo Texts From Last Night (TFLN).

Where will the mobile apps market go from here with respect to advertising, taste, censorship, and ratings for unsuspecting users who like Pepsi and get more than they bargained for?

I’m gonna make a prediction now and say eventually we’ll have something like the rating system for movies (Motion Picture Association of America) or console games (Entertainment Software Rating Board), and I’ll go a bit further and say that will happen within the next year.

Mobile app developers – begin planning now for whatever audience you want – be it ‘R’ or ‘G.’ And beware an inappropriate rating: although the industry as a whole is slow to take a stance, apparently users will not let you get away with misdirection in advertising.