As an example, the Comp Rules were updated February 1, 2012. I know that a reference to "Fateful Hour" was added to 207.2c since I know to search for fateful hour, but how do I know what else changed? Does Wizards of the Coast release an official list of changes when they make a revision?
They put an article in their website written by the current Rules Manager (the main authority in Wizards for Magic rules).
This is the one that corresponds to the February 1, 2012 rules update.
If you search for more articles by the same author, you'll find the articles for older rules updates. Searching for articles of previous Rules Managers is trickier.
I don't believe they release a full changelog every time they make a change to the rules, because most of the time, they're minor patches, like adding/removing creature types, or adding a few new block mechanics.
However, when big changes happen, such as Sixth Edition or M10, they usually discuss those changes in detail, in form of an article, such as this.
I do believe though that a full diff of rules changes would be pretty awesome for the rules lawyers among us. A website doing this could be fully automated, just checking the rules in txt format once a day or so.
EDIT: Googling a bit revealed this gem, which seems to be exactly what I had in mind: A privately maintained, exact changelog between rules versions
Wizards releases what they call Update Bulletins. Within this update bulletin, is a section called Comprehensive Rulebook Changes. You can search for the latter, and eliminate most other update bulletins not referring to the rules changes. Search for comprehensive rulebook changes
Even if Wizards didn't release this list of changes, you could use something like the Internet wayback machine, and a text difference tool (meld, fd, etc.) to find the changes yourself.