AnandTech has a post about Seagate’s new Momentus XT hybrid drive.  It is a notebook hard drive combined with a small 4 GB SSD used for read caching.  The results overall are pretty good especially for notebooks that often don’t have more than one drive bay available to do a separate SSD and HDD combination.  It won’t beat a decent pure SSD, but for normal day to day use it will still be generally faster than any single hard disk drive.

For more information, see AnandTech’s article here.