Writing-for-pay definitely makes a big difference for me. Programming is what I do for a living. I want to do something different as a hobby, so pretty much all the programming I do not-for-pay is one-off scripts to solve a specific problem, and work don't pay me to write entire programs from the ground up.

Yup, same here.

A tentative third for this line of reasoning - could this be 'need for variety' maybe? The more interesting my work coding is, the less desire I feel to hack on my own projects in the evenings - I'd rather spend the time juggling, spinning poi, or other activities that are quite different to what I spent the day doing.

However, coding is quite probably my favourite hobby - so if work is being dull then I start to need to do something interesting with code in the evenings to feel like I'm still indulging.