Im my mind Nokia have endless possibility and endless resource and I have no idea how many man-hours would necessary for a port (I suppose beside the arm processor a tablet is a completely different beast, I was probably confused by the N97 and all the voices like a possible Maemo device).
ok, so having symbian is automagically having, hardware limitation apart, NGage . Get it
I probably tend to consider a good 80% of user tablet an advanced user (probably Im spoiled having spent time on ITT but I bet the percentage of people downloading the flasher is higher than other Nokia products...) and the re-flashing procedure is great (I suppose Nokia experience in the field is gigantic).
From the user point of view (MY point of view obviously) would be just another option that allow access the Ovi services, music shop and Ngage (at least the old) library. And I just imagine how well could look the Symbian logo on the tablet box...

Again my rantles came from my experience: I have no budget for any S60 (just for playing at the 'little developer game') and a sort of SDK running on Nokia controlled hardware is sure appealing for a cheap developer wannabe (like me or indian or chinese, I have no idea how much Nokia ask for their phones there) and since Im mainly a graphic person just having a touch screen with comparable screen dimension seem a way better intermediary 'step' than a normal pc-based sdk (and easier to install and with a fixed speed parameters) and would give a definitive 'personality' to the tablet. I lurked the S60 comunity a bit and I deeply envy the HUGE user base compared to the tablet (I suppose is matter of order of magnitude ).
+ even like a plain consumer device a Symbian OS could still appeal me but yes last barcelona's phones really overlap all the actual tablets killer features (gps, radio, keyboard, processing power, memory, wifi, audio quality). The screen and the speakers are probably still better than any phone around and is just impossible beat the flexibility.
So I can't see any reason why Nokia should not spend a couple of milions of euro to make his user happy.