FLASH memory can be used as data memory, but there are two serious issues you have to take into consideration:

1). Write cycle timing. Although reading is just as fast as from RAM, writing a byte to FLASH takes microseconds rather than nanoseconds. You will have to implement code that checks when the write cycle has finished. This will of course affect the performance of your system.

2). 'Overwriting'. Once a byte has been programmed, it cannot be altered by writing it again with the newe value. Instead, it has to be erased first. Erasing, however, cannot be performed on a byte-by-byte basis, rather, you can only erase a 'block' (or 'sector', as other manufacturers might call it) at a time. These blocks can range in size from 64 bytes up to 16Kbytes, depending on type and manufacturer.

All of a sudden, FLASH doesn't seem such a cheap alternative after all.....

For a reasonably prized NV memory you might want to look at FRAM from Ramtron

Best regards,
Rob Klein.