One perfect example for a 2.5” hard drive that was specifically designed to fit into 3.5” bays is the VelociRaptor drive by Western Digital. It is well-known among enthusiasts and a de-facto standard for high-end client PCs and even some workstations.
Not too long ago, WD decided to add such a 2.5”-to-3.5” drive to its VelociRaptor lineup to be able to serve the SMB and low-level enterprise markets as well. This step made sense, as deployments of the 2.5” VelociRaptor drives into 3.5” environments failed for the reasons explained on the previous page.
However, despite its ability to power demanding business systems these drives are technically not suitable for the enterprise storage segment, as they are only available with SATA 3Gb/s interfaces. In this case, interface speed isn’t a critical factor, but other SAS features like dual ports can be. The VelociRaptor appears interesting for large storage arrays, as it delivers an attractive cost per gigabyte at decent performance levels. However, you will lose all the flexibility that SAS infrastructures provide if you go this route. Please look at the article Next-Generation SAS: 6Gb/s Hits The Enerprise for more details on features and options.