How significant is the new iPad's heat issue?
Not at all. I have noticed that it is warmer when sitting for long periods (i.e. reading in iBooks, playing Hero Academy) but it is not a big deal and not even unpleasant except when the day is hot. If you look at this on a relative scale that I've just made up:

10 - 2007 MacBook Pro 17" playing World of Warcraft
9 - Hell
8 - Dave's Insanity Sauce
7 - Chili con carne
6 - cup of coffee
5 - iPhone 4 in Personal Hotspot mode
4 - cup of coffee when you remember to drink the last bit
3 - New iPad
2 - Old iPad
1 - Fonzie


You get some more perspective.