Install terminal-notifier by "sudo gem install terminal-notifier". Then put this script to ~/Library/Scripts/Applications/OmniFocus and bind a shortcut like Alt-Cmd-Enter to it using FastScripts. Originally based on Curtis Clifton's "What am I doing" script my largely adapted and extended.

OmniFocus Selected Task Notification.applescript

tell application "OmniFocus"
  if quick entry of front document is visible then
		set theSelectedItems to selected trees of quick entry
	else
		tell front document
			tell (first document window whose index is 1)
				set theSelectedItems to selected trees of content
				if ((count of theSelectedItems) ≠ 1) then
					-- try sidebar selection
					set theSelectedItems to selected trees of sidebar
				end if
			end tell
		end tell
	end if
	
	if ((count of theSelectedItems) < 1) then
		display alert "You must first select a project or action." message "Select a project or action to highlight." as warning
		return
	end if
	if ((count of theSelectedItems) > 1) then
		display alert "You must select just one project or action." message "Select a single project or action to highlight." as warning
		return
	end if
	set selectedItem to value of item 1 of theSelectedItems
	set theName to name of selectedItem
	set theId to id of selectedItem
	set flagged of selectedItem to true
	my notify(theName, "", "omnifocus:///task/" & theId, theId)
end tell

on notify(theTitle, theDescription, theUrl, theId)
	do shell script "/usr/bin/terminal-notifier -sender com.omnigroup.OmniFocus -title \"" & theTitle & "\" " & "-message \"" & theDescription & "\" -group \"" & theId & "\" -open \"" & theUrl & "\""
end notify