Several of the Canadian Language Benchmarks (starting around Benchmark 5) require your adult students to understand the inferred meaning in different texts. For example, if your friend says, "Those pants look good, but do will you be able to sit down comfortably?", your friend probably means your pants look too tight. If a commercial for weight-loss pills shows a woman who lost 100 pounds in a week, and the ad says "Results Not Typical", this probably means that those results have a 1% chance of actually occuring with normal use.

I would have assumed that these "innuendos" would be the same in all languages. Assuming they aren't, what is the best way to incorporate them into the curriculum? Is it possible to devote an entire class to "inferred meanings" or is it something students have to learn on a case-by-case basis through the course of their other studies?

It depends a lot on how you typically teach the second language and how your students process this information. If they are used to translate everything, then it will be really necessary to get them out of this frame of mind because some expressions will be completely lost on them if they do word-by-word decoding. In this case it would be necessary to devote at least part of one class explaining what this means and giving some examples. Deal with the rest as they come along in class time.

Have you seen the trick of putting either asprin or alka seltzer in coke on CNN? Check the archives. And take your students outside! Or make popcorn in an air popper so students can see the pop corn pop. Both 'experiments' are great for making inferences!

After giving them some of these examples ask them to look for things like this in conversation or TV. They should write them down and bring them in and you can discuss them and help them to understand.This can be just a short lesson (5 minutes maybe) and you can repeat it using different examples.
Do a 5 minute warm up where you put a few on the board. Have them jot down what they believe the inferred meaning is. Talk about it and move on with the rest of the lesson.

I would think this is a litle bit of both. Give them examples that they may run into on a day-to-day basis(like your first example), and then just tell them to keep an eye out for things that don't exactly seem 100% true. Also, you can do them a huge favor by teaching them common tonal changes and body language cues that go with sarcasm and insincerity, that way they'll learn to tell when someone is insulting them or not telling them the whole truth.

Teaching adults is a bit different from teaching younger students. It is good that you are inquiring about infererrence because it means that you will keep it top of mind. I would personally shy away from devoting an entire class to inferrence unless it is part of an integrated lesson. Perhaps you could use the definition and examples as a warm up to expose your students to the skill you want them to learn before doing a shared read or watching a video clip you have prepared for a different type of lesson. Asking your students the right questions after a reading or viewing a commercial will help them discover and use this skill in everyday life.

Meet them where they are. I would guess that a lot of the students watch TV and enjoy crime dramas. Use an episode of a show like "CSI" to illustrate how the detectives gather evidence and infer what happened.

I have to teach this skill to my 8th graders. I introduce making inferences by drawing the angles on a triangle, but not drawing the sides. I then ask what shape this is, and how they know.