I have been meaning to post this thanks to @JaysonNg for pointing this out a few weeks back, I am unsure when this feature went live since I have been using Globe and 3HKG which are both Apple partner networks in their respective countries. Early this year I recall some folk not being able to access Personal Hotspot & MMS with a non-Globe SIM card. I suspect it was added in 4.3.x
SUN & SMART have been added to the approved list, good job Apple, Sun & Smart.
SUN & SMART have been certified on the iPhone (and I assume iPad) as well. In earlier versions of iOS only Cellular Data meaning internet on the iPhone worked for non-partner SIM cards.
I'm using a SUN Cellular prepaid SIM card because Jayson Ng has already tested these with the Philippine's most EVIL network SMART as you can see, the feature is working but the speed is horrible (budget network SUN Cellular is to blame)
I then plugged in the MMS Settings and sent myself an MMS. Get your settings here http://www.suncellular.com.ph/ecs_configure.php
Previously only Globe SIM cards worked with Cellular Data, Personal Hotspot (the feature formerly known as Internet Tethering) and MMS, 3 of 3 features. SMART & SUN only worked with Cellular Data (1 of 3 features). When I approached both SUN Cellular & SMART Telecom in 2009 they commented that the features were not there because it was locked down in an OS level. I remember bitching about this because I paid full price for my then iPhone 3GS.
I am against locking down devices and suggest if you have the means NEVER purchase an operator locked device. Unlocked means freedom and flexibility.