Just a quick post to help people enable this. Go to http://www.iphone-notes.de/mobileconfig/ and send yourself the tethering settings.

Next go to Settings > General > Network > Cellular Data Network and enter the following settings for MMS:

APN: portalmmm.nl

MMSC: http://mp.mobiel.kpn/mmsc

MMS Proxy: 10.10.100.20:5080

Leave all of the other fields blank, exit the settings app and restart your iPhone. Have fun!