I was wondering about the meaning of:
I am down with something.
Also, I was wondering whether people say:
I am up with something.
If so, what does it mean?
If you are down with something it means that you have knowledge of something or are in agreement with it. I'm down with science means "I am familiar with science" or "science is a good thing." To be down with something is a slang phrase, and not terribly common in formal speech or writing.
You could also speak of having a disease, sometimes preceded by the verb to come if the sickness hasn't yet fully taken hold. eg. I am coming down with a cold. or She is down with the flu.
To be up with something is only used in the imperitive, eg. up with the king! — meaning hey, that king is pretty great. Or: up with ice cream! (Yay! Ice Cream!) You would never say I am up with something. That wouldn't be proper english. Although you could say I am up to something, you sneaky little devil, you. (In case it's not clear, "I am up to something" means "I have an ulterior motive I don't wish to reveal.")
If you are addressing someone directly who is presently seated, you could use a command: up with you! (stand up.)
Other than a literal interpretation
More often than not, these mean "I am in agreement with" but the type of object matters quite a bit.
I am not familiar with "I am up with". It would not surprise me to hear someone using it as a slang version of "I am down with" for emphasis. "I am so down I am up."
More commonly, both phrases can be used with "for" in place of "with":
If the 'something' is an illness or malaise, then "I am down with the lurgy" means that I am suffering from an illness and normally that I am in bed and hence not getting up, so staying down.
I don't think I've ever heard anyone say "I am up with something". In a wholly different context, "I am up on something" means I've studied something and know something about what it is all about.
===== "Up with" =====
"I'm up with James." --> "I'm here with James." (where the current value of 'here' is any place that can be perceived by the listener as either 'upwards' or 'uptown', relative to the listener's current location)
"I'm up with the dawn." --> "I'm risen from bed at the same time the dawn occurs."
"Up with you!" --> archaic form, similar to "Get up!"