One thing to watch out for is to make sure that you respond quickly. I have pretty much the same thing put into my contract at my current company, but I didn't put in a deadline for how long they could take to respond. This resulted in most responses getting back to me many months to a year later and in one case being strung along and never replied to. This has killed many personal projects before they even get started and killed off existing projects I used to contribute to.
I've left one and turned down countless jobs that came with a "we own everything that comes out of your brain" clause. For creative people, it's a huge deterrent. For those who take the job anyway, it dampens the qualities that make one valuable to one's employer in the first place. Good for Imvu to have noticed this and found a way to work with their employees.
At youDevise (https://dev.youdevise.com) we have a similar policy - works great to encourage side projects and attract the kind of developer who likes doing them.
Maybe it's just that I work at a smaller company or something, but we have a de-facto policy like this in place. It's more of a "have lunch with the CEO and tell him what you're working on" then if he's okay with it the company lawyer will write you a letter.