This may be more of a repeat, since predictions of this sort have been done before, but Toshiba is one of the three premier HDD providers, so when it talks about such things, people listen.
Indeed, Toshiba made a statement the other day, one that has to do with hard disk drive, both traditional and more revolutionary ones.
Long story short, the company believes 50% of all HDDs will be hybrid units, with integrated NAND Flash chips, by 2015.
The NAND Flash part essentially turns the HDD into a part-SSD device. With the OS and other frequently used files stored there, the whole system is guaranteed to work faster. Application boot time, for example, is reduced by 40%.
Full SSDs will gain some ground over the next two years as well, but not enough to drive HDDs out.