I wouldnt worry too much about it but if you still are, pay for an hour or two with a Lawyer. The cases where you DO NEED to talk to a Lawyer or your employer are if your current projects compete with your employers. Sometimes this is obvious, sometimes not. Employer makes accounting software and you make first person shooters - not a problem. Employer makes strategy games for the app store and you make first person shooters for the app store - might want to consult a Lawyer. As long as you are doing it on your own time, with your own equipment and it is not in your employers direct line of business - there is nothing to talk about if residing in California.

Additionally if your projects are somewhat aligned with your employers projects get them to sign something before you join - its your best time to negotiate this. Another alternative is to cross out any lines you dont like in your employee contract (although it may ruin the employment contract) - I cross out a significant amount in my employment contracts and usually dont hear much about it.