Launch an app if it's not already launched (and make sure it's focused after launch). Otherwise show/hide it if it's already launched. Could probably be improved...

launch_app.applescript

set appName to "Sparrow"
set startIt to false

tell application "System Events"
  	if not (exists process appName) then
		set startIt to true
	else
		if frontmost of process appName then
			set visible of process appName to false
		else
			set frontmost of process appName to true
		end if
      	end
end tell

if startIt then
	tell application appName to activate
       	delay .1
	tell application "System Events"
		set visible of process appName to true
		set frontmost of process appName to true
	end tell
end if