All the authoritative monolingual dictionaries record two different meanings for 'so-called', even Cambridge, which was quoted by RacDel and papousek in support of the contrary.
1) expressing the speaker’s view that the linguistic description is not common or known to him/her (so-called = what is called by some people or specialists - not 'commonly named' as M-W says)
2) expressing the speaker’s view that a standard wording is used for a real-world thing that doesn’t correspond to its meaning (so-called = alleged)
We can, therefore, safely assume that the two meanings are in common usage and that the LEO entries are correct. It would, however, be helpful for the dictionary user to have two identical entries with expanatory notes:
1) so-called /what is called/ Adj. - sogenannter | sogenannte | sogenanntes (auch: so genannter | so genannte | so genanntes) /namens/
2) so-called /alleged/ Adj. - sogenannter | sogenannte | sogenanntes (auch: so genannter | so genannte | so genanntes) /angeblich/
Both meanings are an expression of opinion, no. 1 with respect to the commonness of linguistic usage, no. 2 with respect to the inappropriateness of the signification the wording suggests. You might call sense 1) neutral use and sense 2) sarcasm or negative use. My impression is that 'so-called' tends to be enclosed in quotation marks when used in sense 1 and less so when used in sense 2.
In the light of the above, I agree with sebastian that A woman receives a Native American smudging ceremony as so-called Dreamers rally outside is not a sarcastic statement, but simply refers to the fact that ‘dreamer’ is not used in the usual sense of the word (sense 1).
On the other hand, I concur with papousek in that the so-called Development, Relief and Education for Alien Minors (Dream) Act, the so-called 'free trade' policies and the so-called 'universal' healthcare are used sarcastically to mean that the phenomena described fail to live up to their names (sense 2). The Act, for instance, is believed not to have the effect that its name suggests, while the existence and usage of the name itself is not challenged. Typically, "so-called" is added when a popular name for an Act of Parliament does not correspond to the official name.
I wouldn't know how to show in the dictionary that the second meaning is used more often in English, as has been argued by several native speakers here, but given the fact that the discussions are part of the LEO dictionary, it would be interesting to see what sentences RacDel corrected because of a wrong usage of “so-called”. This may shed some light on papousek's suggestion that ‘so-called' is best avoided in English.
#10: I just typed "so-called" into the Guardian's own website.
lingua france probably used Google search, which would explain the different results.