Hi,
I'm currently photographing in a few National Parks in the US. Does anyone know what's "the deal" regarding photography releases? I mean – doing things by the boek: do I need a release? If so, how much are they? I tried googling it but couldn't find any info...
Thanks a lot!

As far as I know, what they mean by 'commercial photography' is e.g. a set up with models, shooting a film, advertisement etc, where they'd have to clear off an area for you.
There are lots of USNP photos on iStock, at least some of them don't have releases, and we know how uber-careful iStock are about releases.
Last month (http://www.microstockgroup.com/istockphoto-com/am-i-going-nuts-editorial-rejections) I had a photo including the Statue of Liberty (which is a National Monument as part of the US National Parks service) rejected for the editorial collection and asked to submit it to the main collection. (I had submitted it as editorial because of the yacht and the people, I know there are many images of Lady Liberty already in the main collection).

You need a release for somethings, the Iwo Jima statue (Marine Corps War Memorial) probably the Vietnam memorial, and WWII memorial since they are not very old.