IMHO, the reason why we do not get bored of breathing is because we can do it without having to devote time to it. Therefore, it does not detract from other activities. Same goes for walking, where as I have read, the coordination happens in the lower back and does not involve the brain, or running where only the base of the brain is involved.
I am easily bored with washing dishes because while I am doing that, I cannot be working on a software project. I am also easily bored with tweaking CSS because I see it as inherently unnecessary: one day IE will die and I won't have to waste time on it. Thus, my definition is that boredom is something that feels like a waste of time, a very unrenewable resource.
Exactly; subconscious activities like breathing are the easiest to ignore and therefore we can do other things at the same time. This is probably because they require little though and occur in a part of our brain that it not likely to mess with other activities you're doing at the time (e.g. washing dishes or walking). Therefore, we can multitask with about anything while breathing.
I am easily bored with washing dishes because while I am doing that, I cannot be working on a software project. I am also easily bored with tweaking CSS because I see it as inherently unnecessary: one day IE will die and I won't have to waste time on it. Thus, my definition is that boredom is something that feels like a waste of time, a very unrenewable resource.