The reason Apple dropped model numbers with the iPad has seemed obvious to me for months: because they’re going to be selling the current “new iPad” alongside the yet to be announced iPad mini.
Think about it, if Apple kept the model number then consumers would have to choose between the iPad 3 and iPad mini. What would Apple do for the next version of the iPads? Would we have an iPad 4 and iPad mini 2? Obviously this isn’t a good naming strategy, so Apple chose to simplify everything by dropping the model number from the iPad family name.
Now that Apple has sent out invitations to the new iPhone event with clear hints that it will be called the iPhone 5, the only thing we can be pretty sure about is that Apple will not introduce an iPhone mini/nano alongside the iPhone 5. When Apple finally does drop the model number from the iPhone, expect an iPhone with a new form factor to be introduced within a year.