I suspect that the answers may be more informative of people's different conceptions of what sub-vocalization is, or different ways of measuring it, rather than of the mental processes involved in reading...
E.g. I presume that I can't sub-vocalize while counting out loud, especially counting at a fast speed. I can read, but not write, while counting, which is why I think counting impedes vocalization.
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.