There was such a clamor leading up to the Verizon iPhone launch that you’d think it would be the only thing the world is talking about. Instead, it’s been pretty quiet since the launch, which has everyone wondering, how good was the launch?

If you see things how Marco Arment, founder of the popular Instapaper app for multiple mobile platforms, the Verizon iPhone is selling mostly to existing iPhone customers. Arment used the sales of his own application, which are historically fairly steady, to analyze the current level of Verizon iPhone sales.

Here are the basics from his blog:

Since my ranks rarely change significantly, the resulting sales volumes seem to track the entire App Store’s volume. In other words, since my rank is held mostly constant, but my sales vary, it’s reasonable to extrapolate that trends in my sales indicate approximate trends in the entire App Store market.

The results are fairly obvious: I see huge spikes whenever there’s a new iPhone, iPod Touch, or iPad released, whenever they become available in a major new country, or whenever there’s a major reason for people to buy a lot of them (like the holidays).

Arment hasn’t seen any spikes surrounding the Verizon iPhone release, though. In fact, things have been surprisingly moderate. Arment’s own theory about slow adaptation among Verizon customers seems spot on to me. He thinks most of early adapters are the hardcore smartphone nerds. These are the people that wait in lines and stay up until 3AM to pre-order. These are people who put up with AT&T just so they could have the iPhone.

The next wave of iPhone owners are the casuals – people who have seen the phone and liked it but aren’t in any real hurry to buy one. Casual users always take longer to adapt new tech and the Verizon iPhone won’t likely be an exception.

Bloomberg is reporting that the iPhone may be breaking onto America’s favorite network in January of next year. The report cites two unnamed sources who are, as always, close to the issue.

There has been a lot of speculation about the end of the AT&T exclusivity arrangement, especially when AT&T started offering up iPhone 4 upgrades to anyone that would take one.

The device will be available to customers in January, according to the people, who declined to be named because the information isn’t public. Natalie Kerris, an Apple spokeswoman, and Jeffrey Nelson, a Verizon Wireless spokesman, declined to comment.

The iPhone, which has been the sole domain of rival AT&T in the U.S. since June 2007, will give Verizon a boost in its competition for smartphone customers, UBS AG analyst John Hodulik said in an interview. Verizon customers, who numbered 92.8 million at the end of the first quarter, may buy 3 million iPhones a quarter, he estimates.

For now I’ll remain skeptical, if only because there have been rumors of a Verizon iPhone for years now. Actually, I just don’t want to believe I signed with AT&T when Verizon was just around the corner.