It is special Siemens formatting, and there is a lot of hidden data stored in the background on the card. What Siemens sells as an 8MB MMC turns into something like a 64MB MMC if it is windows formatted. However, at that point it can't be used in a PLC anymore. Siemens says that the their MMC uses higher quality flash memory than consumer grade stuff. Doesn't make it worth the price, but ya gotta have it. I'm not saying its 100% impossible to use a third party MMC, but you would really need to know what you are doing to format it properly. Essentially, you'd have to already be working for Siemens as a developer.
The newer PLC's, the 1200 and 1500, have SD cards that are more reasonably priced. Still nowhere near consumer grade cards, and still required to be a Siemens card, but not nearly as bad. They can also be read in windows, so you don't need the special Siemens MMC prommer to clear it.