With Seagate announced their new ‘Hybrid’ 500gb 2.5inch Drives they claimed it could offer SSD speeds but with only a small premium on top of a normal magnetic hard drive. The drives use inline 4GB flash memory and use what Seagate calls Adaptive Memory. The firmware automatically relocates frequently accessed data and stores it on the flash memory while keeping most stuff on the the drive.
Their marketing pitches substantial improvements to most general usage situations. This video showing off the technology certainly looked impressive.

(via Seagate 500GB XT Hybrid Laptop Drive – Quick test)