From a typical boss-man perspective, your performance is reduced by this extracurricular work. You're probably working late into the evening, and thinking about Ruby during the day. You're probably planning a move to something better.

You can't be fired for that alone, but it puts you at risk of being in the socially excluded, not-getting-the-benefit-of-the-doubt category.

One of the issues with side projects is that admitting to having them breaks the Fundamental Subordinate Dishonesty (http://michaelochurch.wordpress.com/2012/12/12/fundamental-s...). You're "supposed" to be putting all available energy into your job, or at least respect authority enough to put effort into that illusion. Vacation is okay: that's just taking a break. Off-hours work isn't (in typical workplaces).