If the added responsibilities include adding to your skills I'd just go along with it. I'd document everything and how it helped the company. It doesn't feel like it now but the economy will improve which will leave you in a good bargaining position if you decide to stay (ask for a raise) or move on to a place where you can get a good position.
From where I'm sitting this almost looks like an enviable position. I have a full time job but I am not directly managed. I have to get out there on my own to look for projects at work on and through the years I've had problems gaining visibility to get those projects (others in my way, office politics, for example) so in a way I have the opposite problem and trying the best to deal with it.
It is better to be employed and overworked than to be unemployed. Just think of what your experience will look like on a resume when the ecomomy improves!
I am in the exact same boat as you are in...I have been with the company I am working for, for 3 years and keep getting more responsibilities but never get any appreciation for it...no raise, no thank you either. its just expected. I feel your pain. It stinks. Hope things get better for you. sorry i dont have any advise...if you hear any good advice...let me know.
two words. industrial sabotage. make your slave driver suffer, profit if you can.