set resolution to words of (do shell script "/usr/sbin/system_profiler SPDisplaysDataType | grep Resolution")
set {width, height} to {item 2, item 4} of resolution
tell application (path to frontmost application as string)
set bounds of front window to {0, 22, width, height}
end tell
Doesn't seem to work in all applications though...
Update: Sven's solution should work in all applications, even those that don't support Applescript:
set resolution to words of (do shell script "/usr/sbin/system_profiler SPDisplaysDataType | grep Resolution")
set {width, height} to {item 2, item 4} of resolution
tell application "System Events"
tell (first process whose frontmost is true)
set position of first window to {0, 22}
set size of first window to {width, height}
end tell
end tell