Why "iPhone 5" makes sense
Pardon me since this topic has been argued to death, and feel free to ignore this post if you wish.
In light of the Apple event invitation, I'm assuming the next iPhone will be called iPhone 5, and I'm posting this because of two brief reasons I find appropriate to call the new iPhone the "iPhone 5".
Apple is known for upgrading the iPhone's internals without changing the hardware e.g. the 3GS was a spec bump over the 3G and so is the 4S. By having the digit moniker, Apple could signal to the public that everyone shouldn't be expecting a new design every year. This will let Apple call the next iPhone "iPhone 5S". Since Apple produces excellent hardware, there is no need to keep changing it anyways. Calling it the new iPhone will make people expect it to be a sixth-generation / 2012 iPhone, and thus the next year's model will be a new generation one and they would expect the design to change. When designs change, it is a waste for accessory manufacturers to create new products and driving old ones into obsolescence.
I personally find the "new iPad" name to be confusing. The iPad is no longer called the iPad 3 and so when people ask which iPad they own, saying "new iPad/the latest model/the new one with the Retina display" is rather ambiguous. This doesn't apply to the MacBook line though, and that's because people don't generally see much changes year-on-year on these models, and they are classified by the year during which they are launched. In the smartphone space, people expect things to change a lot, and hence the name is very important in distinguishing the models. I'm not sure whether it is just me, but not everyone knows how many iPhones Apple has produced. Calling it iPhone 5 is a natural progression after "4", and compared to "new", it sticks better with the public.