I love this post because I've been feeling a bit guilty about it too lately. As an infant, I wore my daughter on my front and later my back almost all day. I interacted with her constantly. Once she started crawling, and later walking, I spent some time playing with her, but slowly started doing my own thing.
My daughter is 18 months old and surprisingly enjoys playing on her own. She colors, "feeds" her baby, stacks blocks, "cleans" the house, "reads" books, shovels her dry beans, does puzzles etc. I talk to her all the time and involve her in house work (she loves to unload the dishwasher, unload the dryer, put wet clothes in dryer, dust, mop, sweep) whenever I can. We also read, sing, dance, and go to the library, the museum, or playground from time to time. It seems like the only time she really needs me is when she wants to nurse. Recently she has been sick, so of course she has been wanting more of my attention, but this is unusual.
I used to think I needed to sit on the floor with her for hours on end but soon realized that I had way too much stuff to do and she doesn't really need that.
I also recently read The Continuum Concept and it really supported what many of you are saying about letting the kids do their own thing.