You all beat me to this as I just got my first payment email about 15 minutes ago!
It's totally legit and it's using doubleclick tracking codes in the links and a 1 pixel image tracking bug to detect email opens.
I'm suspecting they're doing some market research to see how many AdSense users are:
1) paying attention to Google notifications
2) getting their email, that the current email address is still active
3) checking open counts, there's a 1 pixel tracking bug in the HTML version
4) seeing how many have HTML email capabilities (tracking bug again)
It's also a good customer service thing to remind people they're being paid from AdSense so they might be inclined to put more ads on more pages.