A few of us asked this question a while back: why can't we use Aeroplan miles to upgrade a paid ticket?
From The Boarding Area
"Delta has introduced revenue-based upgrades where miles are used to ‘pay the difference’ between your fare and a higher class of service."
http://viewfromthewing.boardingarea....ased-upgrades/
or at DL's site
"Take advantage of Upgrade with Miles during an original booking directly through Delta"
http://www.delta.com/content/www/en_...ith-miles.html