I just tried logging into my adsense account and got redirected to http://www.google.com/errors/asfe/system_down.html. Anyone have any ideas how long it's been down? I logged in a few hours ago.

this is the right page..
http://www.google.com/errors/asfe/system_down.html

and youre right.. AdSense is down..

<< The Google AdSense website is temporarily unavailable. Please try back later.
We apologize for any inconvenience. >>

adwords ist down, too ;)

though the Help Center works fine ;
http://adwords.google.com/support/
https://www.google.com/adsense/support/

and theyre back on again..

You answer can be found here: http://adwords.blogspot.com/2007/02/adwords-system-maintenance-on-february.html