"No, according to California Labor Code 2870, even if you use your own time/equipment, your project still belongs to IBM if your project relates to IBM's business, or actual or demonstrably anticipated research or development.
So if you think that your project is not close to anything that IBM is or will be involved in, you can tell your manager about it, and if they agree with your assessment, then you can tell me your project and I will try to get it cleared for you. In my experience though, it is hard to get clearance because IBM is involved in practically everything, esp. in the software field."
My manager was ok with it, but the IBM lawyers wouldn't clear it. I doubt they would have pursued any legal actions if I continued working on it, but I figured I'd rather not chance it and let it go.
[1] It was a FOSS Firefox add-on called Map+. IBM had a team somewhere that worked on Firefox add-ons, so even though I wasn't on, near, or had access to that team, it was a no go.