Apple has continued its significant investment in flash memory, with the company announcing Tuesday that it recently prepaid $500 million to Toshiba to secure long-term supply of NAND for its mobile devices, a very necessary component in its smartphones.

The half-billion dollar investment came early on in the September quarter, so it was not officially revealed in Tuesday’s Q3 results. However, Apple COO Tim Cook and CFO Peter Oppenheimer discussed the purchase during the earnings conference call.

“We view flash as a very key component for us,” Cook said, “because as you know we use it so many of our products.” True, and running out just wouldn’t do at all. It appears that won’t happen for some time.

As Cook pointed out during Tuesday’s conference call, Apple products make up a significant portion of the flash memory devices on the market today, including the iPhone, iPod touch, iPod nano and iPod shuffle.

In 2005, Apple paid $1.25 billion in advance to Hynix, Intel, Micron, Samsung Electronics and Toshiba to secure the supply of NAND flash memory. The previous long-term supply agreement runs through 2010, so this secures more supply of memory for the next year or two.