Flying blind here, because I don't have a Lion installation, but I understand that windows in Lion have a new attribute,
AXFullScreen, so scripts should at least be able to detect the full screen condition, and apologize that they are unable to work with it.
Should be something like this (untested) snippet:
Code:
on IsFullScreen()
tell application id "sevs"
tell process "OmniFocus"
set blnFullScreen to false
try
set blnFullScreen to (value of attribute "AXFullScreen" of front window)
end try
return blnFullScreen
end tell
end tell
end IsFullScreen