yak shaving, from MIT's hacker's Jargon File.
Two definitions from the "yak shaving" page in Wiktionary:
a definition in a positive (neutral?) light:
Any apparently useless activity which, by allowing one to overcome intermediate difficulties, allows one to solve a larger problem.
a definition in a negative light:
A less useful activity done consciously or subconsciously to procrastinate about a larger but more useful task.
I don't think yak shaving is a bad thing at all. In fact, I think it's a great thing. Joi Ito agrees.
The only caveat is to learn when to stop so you get something done, especially at work.
But don't make a rule to stop yak shaving: never yak shaving means never learning new things. It means only solving problems with what you already know. You could have a strict division of learning and working: when work, just work, and when learning, just learn. But learning is known to be boring, and yak shaving is the very antidote: it's very easy to motivate yourself to learn when you're learning something that's directly related to a practical problem that you need to solve. Most people get bored when learning because they don't see the applications of the theory, or they wager they won't need it in the near future.