Having read the recent contributions, I still maintain that "stand up to" is not a phrasal verb but a phrasal verb-adverb combination. I hope I can explain my position better this time.
You will notice that the very first word in my first post in this thread is
grammatically. That was intentional. In grammar, words need to fall into neat categories, sometimes counter-intuitive ones. We call those neat categories "parts of speech." There are eight of them: verb, noun, pronoun, adjective, adverb, preposition, conjunction, and interjection. Each of them has properties and characteristics that help us identify it. Every single word in the English language - whether or not it is part of a phrase or clause - simply
must fall under one of those categories.
That said, the term "phrasal verb" is not a part of speech. It is usually used to explain to foreigners why we say "I will pick you up" and not "I will pick you" (at least not without changing the meaning). When identifying parts of speech (and diagramming the sentence, for those of you, like me, who were fortunate enough to have been exposed to that most efficient of parsing methods), such considerations become moot and one limits oneself (please try to overlook the awkwardness of some of my constructions; one tends to wax inarticulate when attempting a grammar explanation) to the eight parts of speech.
Now, going back to our "stand up" example. I will put "take to" aside for now. To make things simple, let's stick with two examples for now.
I stand up when called on.
I stand up for my rights.
Here is how the first sentence parses. I will indicate the part of speech first and an explanation, where applicable, after.
I: pronoun, subject
stand: verb
up: adverb
when: conjunction, subordinating
[I: pronoun, elliptical
am: verb, elliptical, part of verb phrase]
called: verb, part of verb phrase
on: preposition - I will concede here that this is somewhat of an anomaly (after all, quirks come up every now and then) because the sentence in the activevoice would be "when one calls on me," in which the object of the preposition is "me." In this sentence, the object of the preposition has turned into the subject, so the preposition is left "hanging." However, the grammatical logic is there; one might just have trouble diagramming this word. In any case, this is irrelevant to our discussion.
On to the second sentence:
I: pronoun, subject
stand: verb
up: adverb
for: preposition
my: adjective
rights: noun, object of the preposition
Grammatically, "up" is an adverb because it answers the question "where." I stand. I stand where? Up. Again, I wish to insist that it need not sound logical ("up" is not exactly a "place") but it is an adverb because it is qualifying "stand." "I stand up" is not the same as "I stand down," for example. "For," however, is intrinsically connected with "rights." "For my rights" is a prepositional phrase that as a whole acts as an adverb modifying "stand" (or, if you will, "stand up.") I stand up why? For my rights.
If "for" were part of the phrasal verb, it would be an adverb. "For" is clearly not an adverb. It does not answer any of the adverb questions or qualify the verb.
The fact that it changes the meaning of "stand up" has no bearing on its grammatical function. Here is where I feel FFB and I have different understandings of how to grammatically analyze this sentence. I'm not sure, but I think Fenixpollo's understanding of it is also similar to FFB's.
If we look at "take" and "take to," we can see the same phenomenon. "To" is not an adverb describing "take," whether or not it changes the meaning. I take to sleeping in. "To sleeping in" is a prepositional phrase. "Sleeping" is a gerund acting as the object of the preposition. "In" is an adverb describing "sleep" (which makes "sleep in" a phrasal verb, as opposed to "take to," which is a verb-preposition combination.)
Having flushed this out (I think

), I would like to propose my own test:
If the "particle" (to use the term from Fenixpollo's definition) is connected to the following noun in such a way that they express an idea that logically follows the verb, then it is a preposition.
If it is connected to the
verb in such a way that is qualifies it and the noun that follows is an object of the entire verb-particle combination, then it is an adverb, and we have a phrasal verb.
For more examples, compare the following:
I came through the bushes. (no phrasal verb)
I came through for my friend. (phrasal verb)
Basically, the particle in a phrasal verb
cannot have an object of its own.
I picked up my friend. ("up my friend" is not a prepositional phrase).
I ran up the hill ("up the hill" is a prepositional phrase)
I stood up to my enemy. ("to my enemy" is a prepositional phrase)
I stood up for my rights. ("for my rights" is a prepositional phrase)
I stood up my date. ("up my date" is not a prepositional phrase)
I stood up in class. ("up in class" is not a prepositional phrase)
I weighed in with my opinion. ("in with my opinion" is not a prepositional phrase)
I just through of
another test that would work wonderfully, but only with transitive verbs.
Use a pronoun as an object. If you can place the pronoun between the verb and the particle, it is a phrasal verb. If not, it is not.
I picked him up. (works - phrasal verb.)
I stood him up. (works - phrasal verb.)
I stood up him for. (nope)
I stood him up for. (nope)
I stood up for him. (only possibility - not a phrasal verb)
I took him to. (nope)
I took to him. (only possibility)
So - without further ado, I hope I've managed to convince you that
grammatically, "stood up for" is not a phrasal verb, but a combination of a phrasal verb ("stood up") and a preposition ("for.")