1. Whether it's valid or not may depend on prevailing state law that is in effect. Just because you sign a document saying "you own my brain 24x7" does not necessarily mean that it's the case.
2. These provisions are often negotiable. When I worked for Lulu.com, their IP Assignment paperwork was so onerous that they could have claimed to own a fantasy novel that I might write in my spare time. I refused to sign until they made an exception. It took a while, but we worked something out that was reasonable.
3. A lot of times companies hand you a big stack of paperwork to sign, but have no process to audit that it all got returned and signed. Other than tax and insurance related stuff, your best bet may be to put it in your desk drawer and forget about it until somebody asks. There's a good chance nobody ever will. I worked at a job 4.5 years once and never did sign any of the NDA / IP Assignment / etc. crap. It was still in my desk drawer the day I left, right where I put it the day they gave it to me. shrug
4. Always talk to an actual lawyer if you have serious concerns about this stuff. :-)