I totally acknowledge what you guys are referring to. In terms of internal architecture the 3GS was certainly a bigger leap. However this doesn't change my opinion, and I recognise this is a subjective matter.
If the focus is on architecture and performance, the 3GS is a major update. The importance of the compass (which I actually did include originally, boss.king) is also underrated in terms of unlocking A.R. opportunities to developers. However for other users who may perhaps be more focused on the breadth of features, the iPhone 4 added significantly more user-facing benefits.
The retina display is really two major features (IPS and resolution) that could individually be seen as more substantial additions than an oleophobic coating. The front-facing camera, which unlocked video calling as well as developer opportunities, was a major gain for a lot of users. The camera saw by far the biggest upgrade to date, plus the much requested addition of a flash (which is also usable as a flash light, and now with iOS 5 as a notifier of events).
Finally, despite the less dramatic internal upgrade, the iPhone 4 had an equal clock increase of ~200MHz from the 3GS plus a doubling of RAM which is certainly not a minor addition. The iPhone 4 was the first model where it became almost impossible to experience memory issues under normal use.
Any sales writer will tell you there is a massive difference between features and benefits -- it's easy to argue that for many users, the iPhone 4 added more major user-facing benefits than any update prior.
Apologies for the off-topic nature of this discussion! In answering the OP's question, barring leaks the iPhone 5 announcement will be third behind the original iPhone and the original iPad events.
