There's probably more than enough information on this subject, but there's something I ran into;
After the iOS 4.2 update, I also got a iPhone Carrier update through iTunes, and brave as I am, I installed it.
Afterward, several options in the Settings -> General -> Carrier menu of the iPhone were gone. Specifically, the settings regarding the Cellular Data settings, MMS. Basically, all the stuff I had to do manually as described in this blogpost.
Apple must have included the KPN Carrier settings in their Carrier Support package. Checking the default carrier profile location (/Users//Library/iTunes/iPhone Carrier Support/) showed the KPN_nl_iPhone.ipcc profile.
This does make it easier to upgrade to another iOS version, because each earlier upgrade erased the settings I had entered manually the time before. On the downside; The profile could also be updated to remove/disable tethering on the iPhone on the KPN network.
The ipcc file mentioned earlier is the one distributed on (or around) November 24th, 2010.