I have heard the phrase "desde ya" used to mean "in advance." Literally, it means "since already." How is it understood to mean "in advance," or is it simply an idiom with a nonsense literal meaning? Can the phrase mean anything else besides "in advance"?
The DRAE says:
So, the "official" meaning is right now, immediately.
The expression desde ya, however, is used as a closing formula as in desde ya, le quedo agradecido which would mean something like from this moment I am thankful and could be translated (with some freedom) to the standard thanks, in advance; this last use could explain why desde ya can be sometimes understood as in advance, although that is not its intended meaning.
Although Gonzalo's answer is excellent I have something to add about the meaning of desde ya:
Desde ya literally means from now, even if the intended meaning is almost always resumed into this moment it is many times understood as in advance
I personally don't think that desde ya is always a closing formula. It can be used in other cases such as in a letter: