Planning for the weekend and I'm wondering if the exhibit hall, concessions, etc are going to be all or mostly cashless?

That's correct. Pretty much everywhere accepts cards.

I'm not sure why this is off topic though.

But cash will still be accepted, correct?

Very occasionally you will get a discount for using cash; the Auction/Consignment Store gives a 3%? discount, and Steve Jackson Games will do an exact change dance which is really just as good as a discount.  There are always a couple of other random discounts here and there for cash, too. 

Sometimes, vendors will have two lines, one for card and one for cash. The cash line is always shorter.

The food courts in ICC were cashless last year.  Not sure about the concessions inside the dealer hall, they may have been as well.

It might be which vendors I buy from, but for the past two years the only vendor I couldn't use cash at was Dragon Shield. Everything else I was able to pay in cash.