1. must display over whatever the user is doing (sort of like a Windows elevation prompt) and
2. must not be handed to userland in a way that it could be automatically recorded or persisted, other than by the user's explicit action.
Giving SpringBoard the ability to render SMSes is really the only way to implement Flash SMS in a way that adheres to its semantics.
There are other types of SMSes as well—WAP Push messages, for example, or WMI (voicemail indicator) Activation messages. Heck, your carrier can even use SMSes to directly write data to your SIM card. These aren't so clearly a layering violation as Flash SMS, as they are just pure OS-layer concerns; but if you're already implementing a kernel library for "SMSes triggering OS-level functionality" for Flash SMS and the like, you may as well put the code to handle these cases in there as well.
