agree with above, and just went through this a week or two ago. i got a letter from my company saying they have no ownership stake in what i'm doing (an extra complication was that i'm working part time in the office and part time on my idea as part of an agreement we struck.) it cost me like $1,000 out of pocket to get an attorney and a letter drafted saying my employer has no "right, title, or interest" in my new venture and related patents/applications/copyrights/etc., but in the grand scheme of things it's a no-brainer to make sure the IP around the idea/new business is clean.

if you have questions about this shoot me an email at houston at alum.mit.edu ... it was a pretty straightforward process.

-drew