Several hours of downtime experienced Friday might have cost Internet retail giant Amazon (www.amazon.com) millions in lost sales.

One of the largest e-commerce site on the Internet, Amazon appeared unavailable for part of the day, its .com web address returning a message saying "Http/1.1 Service Unavailable."

Some reports say the outage began at approximately 1:30 p.m. Eastern Time, and it still appeared down at roughly 3:30 p.m.

There is no question the downtime cost Amazon considerably in terms of sales. The e-commerce portion of the company dealt with 58 million unique visitors in the US in April, according to Comscore. Estimates from Ad Age suggest the company could do as much as $1.8 million an hour in business.

The actual retail losses are as good as impossible to calculate, since at least some portion of the company's sales go to a relatively loyal base of users that could easily make the same purchase tomorrow. And an hour or two of downtime is unlikely to turn those users off using the site entirely.

Customers of Amazon's hosted services, however, might be more easily swayed by some downtime. Amazon, of course, hosts its own infrastructure. And as such, the site is a bit of a proving ground for its Amazon Web Services products, which provide on-demand processing and storage for application developers.

It's not immediately clear whether there were any major service interruptions to the AWS products. According to report posted on the seattlepi.com news site, a developer message board for the Simple Storage Service had a user post at 5 a.m. (presumably Pacific Time) that a "massive internal server error" had started about 35 minutes earlier. Someone from the AWS team responded that they were investigating shortly thereafter.

An hour or two of downtime is a big surprise, coming from one of the biggest retail sites on the web, and the popular micro-blogging service Twitter has apparently been, well, abuzz with posts curious about the outage.

We'll follow up when more information about the outage becomes available.