Specifically what does this mean? If a director is "attached" to a project does that mean he's signed or just interested?
Attached means that some talent or other -- a star or a director or whatever is "linked" to a project.
Sometimes those things are memorialized in writing (always a good idea) -- often just a letter of intent will do, and sometimes not. It definitely doesn't mean that someone is "just interested." If somebody was just interested in a project and you said that they were attached, you would definitely be misrepresenting them and yourself and you could get into trouble.
The point is though, attachments are sometimes good and sometimes bad, depending on the attachment.
It's very common, for instance, for producers to try to get themselves attached to a project as it wends its way through the development process, often for very tenuous reasons. Sometimes, all a producer brings is *access* to a star. Not the star himself -- but just access -- the ability to put someone in the room with the star. But in order to provide that access, they get "attached" -- and that attachment carries with it a substantial producing fee. And they get paid that fee whether the star in question actually agrees to be in the final movie or not.
So they say, in reference to attachments -- some are balloons and some are ballast. That is, some will lift your project up toward production, others weigh your project down. The trick is knowing, as you make your way along, which are which.
NMS