HI,
Just tweeted about this, and wondering how you would rule it:
The paladin feature divine smite specifies that you must use a 'paladin spell slot'.
On Aug 22, Mike Mearls tweeted that the word 'paladin' in that sentence is reduntant.
On Novemner 24, Mike Mearls tweeted that the feature specifies 'paladin' spell slot which means that you cannot use a pact magic spell slot to use divine smite.
Now I am confused. 
Can anyone clarify? The only way I can see to reconcile thw two tweets is that pact magic spell slots are entirely different than "normal" multiclassing spell slots. So a multiclass Paladin/Wizard can use any of the muticlass spell slots to use divine smite. But a multiclass paladin/warlock can only use paladin spell slots to use divine smite.
Is that how you see it?
Thanks,
Neil
You are correct.
Spell slots =/= Pact Magic Slots.
They are mutually exclusive features, with most spellcasters having the former, and pact magic slots being exclusive to the warlock. A mc paladin 4/sorcerer 2 can use any of their spell slots to fuel their divine smite as they combine their spellcasting levels together for the purpose of determining spell slots - in this case granting them the spell slots of a 4th level spellcaster (4/2 + 2).
. A mc paladin/warlock however, does not combine their paladin and warlock levels together for the purpose of either class feature. A paladin 4/warlock 2 has the spell slots of a 2nd level spellcaster (paladin), and the pact slots of a 2nd level warlock.
Like many other posters on this forum, we scratch our heads when it comes to the rules for mc warlocks, especially in light of the stated design goal of not hosing multiclass spellcasters. While they accomplish this goal in respect to cantrip scaling, spells cast at a higher level, and multiclass spell slots - they dropped the ball in respect to multiclass warlocks. My guess is that playtesting revealed that allowing class levels to stack for the purpose of spell slots and pact slots was overpowered, and/or overshadowed the pure warlock.
Thanks for this. I'll have to stop being excited about using spell slots that recharge on a short rest, in order to use divine smite more often! ;)