Running an intel graphics chip may have something to do it. I know with my own chip, mint boots in low brightness, but upon login it seems to reverse the brightness setting so that it's back up to full.
For me, I can adjust the brightness by going into the power management preferences and fiddling with the brightness slider. But the brightness hotkeys certainly don't function.
The only distribution I've seen which works properly in this regard is Fedora 11. So I'm assuming it's one of those issues which is dealt with by upstream progression, and Fedora is (as usual) one step ahead of everyone in adopting the measures. Hopefully that means we'll see a fix either through the repos, or in the next version of Mint.
...loving life for Christ's glory...