use the Command-Tab shortcut to get to the application. With the application selected press the Option key and release the Command key. One minimized window of the application will become un-minimized (if you have more than one minimized application window, you have to un-minimize the other windows the old fashioned way).
Alternatively you can use a utility like FastScripts to assign a keyboard shortcut to an Applescript file. I have set up
Cmd+Option+M to run the following script which seems to work fine for most applications on OS X Lion:
tell application (path to frontmost application as text) try set miniaturized of windows to false -- most apps end try try set collapsed of windows to false -- Finder end try end tell -- Hack from willshouse.com to make sure Finder really does unminimize windows if (name of (info for (path to frontmost application)) as text) is "Finder.app" then try tell application "Finder" set collapsed of windows to false -- Finder end tell end try end if