I've been suffering from a case of unmotivationitis lately. The difficulty is distinguishing serious unhappiness from the natural vagaries of graduate student moods - because you need to know the root cause to treat it effectively. (If simply saying, "Hey! Work harder!" was effective, there wouldn't be a problem, after all, and graduate students would be marvelously efficient.) I think I figured out where my slackitude is coming from, so I'm partway to a cure. I also had a chat with my advisor today, and he was helpful, although not in ways that were entirely expected.
The biggest issue is that I'm making the transition from "doing stuff for other people on deadlines" and "having to do stuff on my own for long-term rewards." It's easy enough to get things done for classes or grant deadlines. It's harder to prioritize journal articles and research projects that are due whenever they're done. After all, the rewards (and penalties) for getting classwork done in a timely manner are pretty immediate. The rewards from getting a journal article submitted are more long-term and nebulous ... someday you will be considered for more jobs, tenure, respect, etc. As you can see, though, long-term doesn't mean unimportant! I think I may have come up with a strategy to help me get going again, and I threaten to keep you updated on it.