Since "Clueless" and before that, since "Valley Girl", it has been "like," everywhere. It's "like"; it's taken over normal sentences and caused a lot of English teachers to have some conniption "like" fits. Like a lot of words, it's wandered away from its "like," original usage and "like" is now placed in any position in a sentence where people feel "like."
And, I guess herein lies "The 'like' Question": are there rules or something that govern these seemingly abberant uses of "like"? Just examining my own "like" usage in this post so far it, "like," seems able to modify either a Verb Phrase (VP) ("so, do you like, like like him?") or a Noun Phrase (NP), and I know that it can appear, ("like" seemingly independently) at the beginning of a sentence, as in "Like, I know you're busy, but do you want to like, go to the movies?" This example also shows it, "like", splitting an infinitive, a place usually reserved for adverbs.
"Like" is used in an informal speech style, and is often associated with the types of "Clueless" girls that don't take themselves very seriously. But, i've also heard very serious male philosophy students say things like, "Like, what I think Derrida is, like, meaning to say is that, like, because the tribe had naming or whatever, they had, like, differentiation, and thus, like, a type of, you know, writing, and so they weren't like, pure or whatever." Each instance of "like" in this sentence mitigates the impact of the following phrase or word. It is the stutter-step that indicates the speaker's insecurity in the use of a word, somewhat like scare quotes.
When "like" is used to introduce a quote, the way i did in the last paragraph, or as in "He was like, 'are you, like, busy?' And I was all like, 'no, why?'" It indicates a paraphrasing. The speaker may not remember exactly what was said, but something like it.
Though the meaning of a sentence might not change with the "like" addition, its tone certainly does. It often precedes the "big words" or important concepts in a sentence, and allows the speaker to, like, fudge their usage slightly.
Here are some links to theories on the subject:
Language Log: A psycholinguistic discussion
Everything2: Like as deponent verb
Agoraphilia: Like's generation gap
Tangent's Mailbag (under "Social Issues"): a misguided attempt at categorization, and juveniles abuse language
If anyone knows of anything else out there, please let us know!