Funny thing about Planet Hulk.
The meta history of Planet Hulk is that it came about because they wanted to give an explanation for why Hulk was not involved in Civil War. The answer was that Hulk was shot into space by The Illuminati after a big rampage in Nevada because he was such a huge threat to humanity. What happened in Vegas did not stay on Earth.
That's such a weird and crazy explanation, when the obvious answer is 'we're already at risk of setting New York on fire, lets not bring the big green jug of kerosene'. Banner, Steve and even Tony all likely know that.
Most of the heroes were pulling their punches in Civil War. Hulk is not always great at doing that. Registration is an interesting topic for many heroes, and Hulk is no exception to that. Given how Hulk has been persecuted by Ross and other government officials (and given how their efforts to control him usually goes), he'd probably wise to go into hiding. Banner would love to be under control, but it never works out right for him.
Planet Hulk and WWH are two of my favourite storylines of the 2000s. But I think its amusing they had this wildly creative event comic for something that could easily have been explained in three panels stating the obvious. Where is Hulk during civil war? Anywhere where he won't rip another superhero's head off.