Ask HN: Working on my own stuff while working at a startup

I'm interested in starting a quasi-legal, but obviously not discussed by legal minds, discussion about work-related, free time projects.

I work as the lead developer for a startup. I love to code, I love to solve problems and very much love & enjoy my job. From time to time, though, I work on projects outside of the office. The company owner has made it clear that as long as it doesn't negatively impact the job, there isn't an issue. Any agreements I have signed don't have any restrictions on not working outside of the office.

So here is my question: Is it "kosher" to go about working on semi-related projects and putting them under my name? For example, a "graph data format" for use with ProtoVis <http://vis.stanford.edu/protovis/> for passing graph data from the backend through to the JavaScript graph generator. Did I build it for work? Not directly. I see the need and am developing it. Will it make my life at work easier? Most likely as it so happens I have decided to use ProtoVis in a work project.

So, HN'ers. Where and how do you draw the boundary line for working on projects related to your work? Aren't so many businesses built on the idea of seeing something that can be made better and doing that?

Thanks!