Many Apple (AAPL) watchers are finding it hard to believe that summer might come and go without a new iPhone, as some are reporting.

But to Marco Arment, the rumors make a lot of sense.

In a Thursday blog post entitled “Apple’s boring hardware updates,” the creator of Instapaper and co-host of Build and Analyze argues that refinements in iOS hardware are reaching a point of diminishing returns, where minor updates — like a new color or a new chip — can be handled with a simple press release.

“Like the MacBook Pro,” he writes, “Apple has refined the iPhone and iPad designs almost to the point that it’s difficult to think of how they’d meaningfully improve them without major underlying changes (such as a breakthrough in storage or battery technology).”

Besides, he writes, waiting until fall to unveil the iPhone 5 “explains a lot about some recent inconveniences that we’ve mostly been ignoring because they didn’t fit into our expected Apple-view.”

Specifically:

• Apple released a significant change to the iPhone 4, the Verizon CDMA model, mid-cycle. Why release such a major product that’s only intended to sell for five months?
• Apple’s most recent statement on the white iPhone 4 was that they still intend to ship them “this spring”. We assumed that they were just stalling, hoping people would forget about the promised white iPhone 4 until the next iPhone is released. But assume it’s not just stalling — again, why manufacture and ship another product that’s intended to sell for only two or three months?
• The annual fall iPod event is less important every year as traditional-iPod demand shifts to the Touch and iPhone, and the updates to the traditional iPod get less drastic and far less relevant. It’s nice to see the Touch’s updates at this event, but since it’s always the same core hardware as the iPhone announced a few months prior, it’s not particularly surprising or newsworthy.

So Arment would not be surprised if the headliner for Apple’s fall product classic — the special event at which it introduces the new gadgets it hopes to sell by the millions in the coming holiday season — is an iPhone and not an iPod.

Who knows, maybe by then Steve Jobs will be back from his medical leave.

Also on Fortune.com:

• About that iPhone 5 delay
• Why Apple’s developer conferences sell out faster every year
• How many iPads did Apple sell?

[Follow Philip Elmer-DeWitt on Twitter @philiped]