That would be a violation of the 3GPP standards, and the iPad would not be approved to access GSM-based networks. Not to mention that a software-programmable IMSI would be a big security risk.
I don't get why people think Apple had to invent a "magic SIM" for this. Global SIMs for business travelers that work just like the Apple SIM have been around for a long time. The only new things are the business contracts that Apple has made with the carriers and a UI for activiating the SIM directly on the iPad.
In the end, it doesn't change much from a customer perspective since you could always easily switch carriers on the iPad. It's just getting a bit more convenient, since you don't have to obtain physical SIMs for the participating carriers anymore. This would be very handy particularly for international travel, since it's not always easy to find and obtain a suitable local SIM.
But we'll all see how it works next week.