Most employers take the position that everything you do in the space is logically competitive. Recognizing that, we, Google, make it very easy to get permission to open source things, but under Google copyright.
We have a process for releasing under your own copyright, but it is not as flexible. That said, we've cleared about 200 (75% of those requested) projects in the last year for copyright release.
It's easy to get knee-jerk about this and get upset, but recognizing the nature of IP in a company and acting on it is infinitely better than pretending the problem doesn't exist and then, in the future, retroactively claiming copyright when convenient. The latter leads to lawsuits and unfair restraints on creativity and competition.
This system works. We've released 10s of millions of lines of code into thousands of open source projects. It protects the employees and it protects Google and the price is a bit of complexity and the odd thread like this.
Also, thanks to this system, Googlers can work on Google equipment and during their 20% on these kinds of projects, which is clearly useful and recognizes that people who work on open source almost universally do so without regard to the machinery or network they use.