But in the Beta I discovered that access to Deepholm was by level-restricted portal only, which seemed to derail my power-leveling recommendation. I'm happy to report that this is not the case as of this writing (these things are always subject to change). Yesterday I leveled mining all the way to 525 on my level 82 rogue. I bypassed Deepholm completely and went straight from Mount Hyjal to Twilight Highlands to hit the Mining cap. Since the Highlands are right next to Loch Modan, anyone can get there via flying mount. I had no problem getting around and aggro was minimal.
Just to confirm the finding, I recruited a level 80 death knight to run through the same routes this afternoon. She was able to easily level mining to 525 with only a few aggro attacks. I didn't personally handle this task, but my recruit estimates that she spent about an hour on the process which included a bit of "piddling around". I assume she means exploring and that no actual puddles were created.
So there you have it, it's still possible to level Mining quickly for those who want to get a jump on the competition when Cataclysm is Released. If you have any other Mining questions, feel free to leave them in the Comments!