ipad
While looking at iPad models, I notice several showing as (MM).
In particular, the 4th generation iPad mini with Cellular has models A1454 and
A1455 with the latter being labeled (MM). The same exists for A1459/A1460 on the larger iPad with a Retina display.
What does MM represent?
iOS 5 comes in different "flavors" for each device. iOS is stored on the computer you sync to as an .ipsw file. These files are located here:
~/Library/iTunes/iPod Software Updates/
~/Library/iTunes/iPad Software Updates/
~/Library/iTunes/iPhone Software Updates/
Each device's "flavor" of iOS is optimized and only contains the code needed for the device it pertains to.
There is an app called Air Display that will allow you to do this. It also recently got retina display support for the new iPad.
Some notes:
MacRumors and 9to5mac.com discussed the upcoming retnia support update coming soon last week. They issued a press release of some sort last week, many blogs covered this upcoming feature. They also covered this on their blog and how it will work.
Update - it looks like this feature is now available in version 1.6. More on MacStories as well.
Best Answer
I do not have a direct link or citation on/about Apple to prove my conclusion. But the most logical answer I could gather by reading several sites is that MM would stand for "Multi-mode", signifying that the iPad supports not just CDMA and GSM, but also multiple LTE implementations. The term "multimode" or "multi-mode" is used to designate chips/devices that support multiple, disparate communication technologies (a device with WiFi and Bluetooth can be called multimode too, but we're talking only about cellular here. Plus, all of Apple's iDevices have had WiFi and Bluetooth in all models).
Only the GSM + CDMA + LTE models that support multiple carriers have the "MM" tag.
Example: The 4th generation iPad and the iPad mini have models that support CDMA/GSM/LTE on Verizon and Sprint. So they're marked as "Cellular (MM)" to give them a more substantial (and carrier neutral) name.
The GSM + CDMA + LTE models in older generations that support only Verizon have a "VZ" tag instead of the "MM" tag.
Example: The 3rd generation iPad, the first iPad to come with support for both CDMA and GSM on a single device, has a CDMA/GSM/LTE Verizon model, but it does not support Sprint. So it's marked as "Cellular (VZ)" to signify that it's tied to Verizon (and cannot work with another CDMA carrier like Sprint).
In Apple's current terminology:
"Cellular" implies GSM+LTE
"Cellular (MM)" implies GSM+CDMA+LTE (across CDMA carriers)
Note that there is no "CDMA only" version of the iPad after the iPad 2, unlike "GSM only" versions that have been and continue to be available.
You can come to similar conclusions by comparing the cellular support across iPad models/generations and how they relate to the chipsets used, the carriers supported, and their technology implementations.
Here are some links related to the iPad specs across generations and multi-mode (as it applies to cellular technology) that would help understand my conclusion better.
Technical specifications for iPads of different generations (pay attention to the technologies/carriers supported):
1. iPad (4th Generation) - Technical Specifications
2. iPad mini - Technical Specifications
2. iPad (3rd Generation) - Technical Specifications
3. iPad 2 - Technical Specifications
Links related to multi-mode (read them completely or search for "mode" to go to multimode related snippets on these pages):
1. Multi-band and multi-mode phones, Multi-mode and multi-band mobile phones
2. Cell phone bands and modes
3. Qualcomm Fifth Generation Gobi Platform
4. A rare look inside an LTE cell site
5. Sprint - multimodal hardware rollout
6. Intel previews multimode LTE chips