The most important piece: if you did it once, you can do it again. Remember this.

Now the details:

http://en.wikipedia.org/wiki/Work_for_hire

You should read your contract with them. As a rule of thumb, if you are in an "innovative" job, that is you were hired to invent, than they might have a claim. Nevertheless, if you want to keep the job you will have to settle, but they don't know it? or do they? I don't know the details... Can the project run at all w/o you? Are you essential to maintaining it? You can negotiate anything. Cash, future share of an exit/profits etc. Look at their demand as a starting point of the negotiations.