There were only four customers in the queue to buy an iPhone 3GS when I showed up at Apple’s AAPL flagship Fifth Avenue store in Manhattan Tuesday afternoon for what I’d been told would be the lunch-hour crush.

This is where I’d hoped to see iPhone demand collide with iPhone supply. I saw nothing of the sort.

Although Apple’s availability widget shows red “sold out” lights for selected models in every state where Apple has retail outlets (see here), there seemed to be plenty of product on hand today.

Less than three weeks after the launch of the iPhone 3GS, the lines that once snaked in front of the big glass cube had dwindled to the point where the customers that showed up could fit comfortably between a few retractable belt posts within the store itself.

Justin, one of the teal-shirted Apple employees manning the stanchions Tuesday, said that the big lines had disappeared within a week or so and that what crowds there were now didn’t start to gather until later in the day — around 4 p.m. As I watched for a half-hour, the queue grew until it held a dozen people. At that point it seemed to reach a steady state, with customers leaving with their new iPhones in hand at roughly the same rate as new ones joined the line.

The rest of the store, meanwhile, was hopping with its usual mix of office workers, tourists, foreign visitors flush with Euros, parents with kids in tow, and out-of-school students hunched over the laptops on display. I estimated the crowd at between 200 and 300.

There were no lines in front of the nearest Sprint S store, 10 blocks down Fifth Ave, and no crowd inside, either. It’s not a fair comparison because there are thousands of Sprint outlets and only 240 Apple Stores, but this Sprint store was very quiet. Of the two staff on duty, one was dealing with the sole customer, and the other was reading a book. She looked up to confirm that yes, they did have the Palm PALM Pre in stock.

See, by contrast:

[NOTE: An earlier version of this story confused Sprint with T-Mobile. Apologies to anyone who was mystified by the switch.]