I'm slowly becoming convinced that Writer's Block, like ADD, has become a blanket term for a series of symptoms that do not necessarily share a common cause.
When people talk about Writer's Block, they usually mean (duh!) that they can't write; that they stare at the page, and they know they ought to put some words on it, or change some of the words that are already there, and they maybe even start to do that a few times, but they can't seem to find anything that sounds right and they really just aren't making any progress no matter how hard they try.
So what I've noticed lately is that there seem to be several strains of Writer's Block. There's the SleepOnIt strain, which goes away by itself by morning. There's the WritePastIt strain, which refuses to go away no matter how much sleep you get, but somehow unclogs itself if you either (a) skip to a later point in the text or (b) write absolutely lousy prose just to get through the scene. There's also the tough-nut BrainStormIt breed, which won't go away unless you clobber it with outlines, note cards, really big wall calendars, and several late-night discussions with friends.
I'm convinced that these are not just different ways of solving the same problem, but customized solutions to very different kinds of problems that simply happen to cause the same symptoms. Hence, we have:
SleepOnIt Block -- caused by general fatigue, an unusual bout of creative exertion, or a jumble of distracting Real Life events, all of which prevent the muse from working at top efficiency.
IgnoreIt Block -- caused by a couple of subconscious connections that haven't clicked into place yet. Working on another project (such as weeding the garden or vacuuming the cat) gives the subconscious time to work without a nosy Conscious Mind peeking over its shoulder.
WritePastIt Block -- caused because the optimal implementation of the current scene depends on the way events will unfold later on, and you haven't figured out those events yet. Writing past the block area allows the rest of the story to solidify, which in turn makes it easier to work on the problem area.
BrainStormIt Block -- caused because the story and/or world and/or characters have become so complicated that you no longer have a clear understanding of all the causalities and connections.
BackTracking Block -- caused because, somewhere earlier in the story, you goofed up and took things in a direction they weren't supposed to go. Your subconscious is aware of this, and screams bloody murder.
Of course this is a simplistic rendering of a very complex phenomenon, but I find it useful to classify different types of Writer's Block this way. It helps me think about the situation and what, exactly, is causing the block, which in turn helps me know which remedy to apply.