I don't sub-vocalize, as far as I can tell. I tend to read by just looking at the first letter of a word and then the shape of the word, rather then by reading phonetically; that's just how I taught myself to read, even before I stated learning to read in school. The upside is that I read much faster then most people seem to; the downside is that I'm terrible at spelling, and I tend to be bad at pronouncing words out loud if I've only read them and not heard them.
In fact, I've noticed before that I often think in written English rather then in audible English; I sometimes "see" words when I'm thinking to myself instead of "hearing" them.
Most people read by recognizing whole words (mainly using first and last letters and the overall shape), so I don't think that's much evidence about whether you subvocalize. It is also probably not much connected to your spelling ability. That's not to say that I doubt your introspection about not subvocalizing any more than other people's introspection.
http://en.wikipedia.org/wiki/Subvocalization
I'm curious about how often or to what degree visitors to this site subvocalize as they read. I was originally interested in reducing subvocalizations as a way to increase reading speed, as the idea is mentioned in multiple pieces I've read about speed reading.
The Wikipedia entry seems to focus on subtle throat and muscle movements, but I'm more interested to know if you "say" or "hear" the words in your head as you read.
Since reading about subvocalization recently, I seem to notice that I "say/hear" what I'm reading quite frequently. I'm not sure if this is causal (in the way that the command "don't think of pink elephants" obliges you to do so), or if I just notice it more now, or both.
When I'm very engrossed in a book either I don't notice the subvocalizations or they stop happening, so seems that it could either be a cause or a symptom of distractedness.
In the comments, please describe your mental subvocalizations (or lack of them) and if they are related to how engrossed you are in the book. Any other comments relevant comments about speed reading or subvocalizations are welcome.