/clearfocus [target=focus, dead][button:3]
/focus [button:1/3, target=focus, noexists]
/cast [button:1/3, target=focus][button:2] spell
If you left-click the icon, the macro will cast the spell on your current focus (without changing your current target). If your focus is dead or doesn't exist, the macro will set your focus to your current target and cast the spell on it.
If you right-click the icon, the macro will cast the spell on your current target, ignoring your focus.
If you middle-click the icon, the macro will switch your focus to your current target and cast the spell on it.
Basically, this macro allows you to designate a target as your focus, and always have your crowd control spells work on that target. So you don't need to find your target before re-sheeping or whatever CC you are using. It should have a decent amount of flexibility with regards to CC'ing a different target in the middle of a fight.
The only paladin spell I'm using it for is Turn Undead. I imagine it will be a lot more useful on my warlock.
Edit: Patch 2.1 has apparently changed how conditionals are handled. I've edited the macro above.