Seagate's new Momentus 5400 PSD 2.5" hybrid HDD is available today.
The drive is available in 80, 120 and 160GB capacities and is a combination between a regular HDD and a SSD.
Each of these hybrid HDDs features 256MB of flash memory which will be used to make the drive more quiet and to lower the power consumption. Additionally, the flash memory should also improve performance a bit.
SSD are better than these hybrid HDDs but the problem is that flash memory still costs a lot. It's a lot cheaper to slap 256MB of NAND into a HDD than to create a SSD with a decent storage capacity.