Personally, I know nothing about Serial ATA, and what makes it so much better than normal ATA. Maybe someone can enlighten me?
Anyway, 2.5" means o­nly o­ne thing: notebooks.

Fujitsu Corporation announced that they will begin shipment of the 2.5" Serial ATA supporting hard drives, the "MHT20xxBH" series, in the latter part of April. Three models are available: MTH2040BH, MHT2060BH, and the MHT2080BH, with 40, 60, and 80 gigabyte capacities.

This is the world's first 2.5" hard drive supporting the serial ATA interface. When compared to the current model, the "MHT20xxAH," this drive has 1.5 times the transfer speed - at 150MB/sec. The moter uses liquid bearings, and has a rotation speed of 5,400rpm. The drive has an 8MB buffer. It also supports Native Command Queuing.

The main unit is 70 x 100 x 9.5mm (W x D x H), and weighs under 99g.

Toshiba hopes that the 2.5" Serial ATA interface will be employed in the mobile PC market from this point o­n. They are aiming for sales of 2,000,000 units in the first year.

Original Article:
http://pc.watch.impress.co.jp/docs/2004/0105/fujitsu.htm

News Release (Japanese):
http://hdd.fujitsu.com/jp/catalog/mht20bh.html

News Release (English):
http://pr.fujitsu.com/en/news/2004/01/5.html