I think it should be a common rule of etiquette not to ask about someone's availability without first saying why. Nothing frustrates me more (I'm exaggerating) than someone asking me something like, "Hey, so what are you doing next Thursday?"
Whenever I get asked that question, I immediately rack my brain for a vague answer. Why? Because whether or not I'm busy depends on why you're asking. Want me to help you haul stuff to the dump? Oh, I'm sorry, I already have plans for that day. Want me to fill in for you on a $40/hour shoot? Sure! My schedule's free and clear!
Most of the time, my answers have been along the lines of "Oh, I have a job penciled in, but nothing's definite yet" but you can only use that so many times. Plus, work's been notoriously slow these days, so I'm getting even less mileage out of it.
So help me out here folks. A simple sentence like "What are you doing tomorrow? I need someone to _____" would avoid a lot of future awkwardness.