You have a number of options:

  * just do it, and ask forgiveness or deal with the consequences.
  * ask a lawyer to look at your contract and state law and see if the company has the right to what you do off hours on your own equipment
  * try to sell the side project as beneficial to the company (cross training, extra publicity)
  * do a side project, but expect to throw it away
  * pick different ways to do a side project--there are many way.  Contribute to open source via QA, documentation, user support.  Do user group talks.  Reach out to authors in the space that you are in and offer to review technical books.  Join an email list and answer questions of other users.
  * walk--find a different boss in the same company who will let you do this
  * walk--find a different company

What you choose depends on what your side project is, what your skills are, how much you care about the side project, how monetizable the side project is, and, most of all, what you are trying to get out of your "serious side projects".