AirDrop will NOT work on anything prior to the iPhone 5 or iPad 4.
I'm sure it's in the hardware. As stated here, AirDrop only works on a limited number of idevices that are currently selling.
This is an oversight on Apple's behalf. What they have done is link 4S to iOS 7, which, implemented here in summary mode, displays a generic introductory paragraph to iOS 7, not specific to the 4S at all. This kind of summary can be misleading for those purchasing the 4S, or at least cause confusion for posts like this.
I do think AirDrop will be in future Apple ads, it will sell iOS devices, however, it is not available on iPhone 4S, iPad 2, not sure about iPad Mini...it works on currently available iPod touches. (A6 processor or better (except iPod touch), whether that has to do with the wifi or bluetooth that comes with or the processor itself).