Fortune reports that Apple retail store stocks of the 16 GB white iPhone 3GS model have fallen precipitously low early this week. At present, only 10% of Apple’s over 200 retail stores have the model currently in stock.

Apple has been having trouble keeping the entry-level white iPhone in stock since the new 3GS went on sale two and a half weeks ago. But as of Tuesday morning/afternoon, it has all but disappeared, and it’s not clear whether demand for the white 16GB model is unusually high, or if Apple just isn’t making enough of them.

Most Apple retail stores currently have both the black and white 32 GB models available, while the black 16 GB is available in only about 35-40% of stores.

In late June, Apple posted an online availability tracker for the new iPhone 3GS, updated hourly with stock levels at each of its retail stores. Supplies of many models tightened early last week, but rebounded later in the week, and we’re expecting the same here. Early next week things should look up.