There is nothing preventing 2.5in drives from being used in desktops. But yess, 3.5in ones exist.

Here is a picture with one

http://www.tomshardware.com/reviews/ssd-performance-power,2279.html

The read rates on SSD's will blow away traditional hard drive, but most of the time, their write rate are not impressive.

On the other hand, most of the time, SSD's are used for OS and page fileing so read speed is far more important.

My only fear of SSD's is degredation over time, but I hope modern technology has made them resilient enough to stand constant writing due to page fileing.

Here are some benchamrks if yopu are interested.

http://www.tomshardware.com/reviews/hdd-terabyte-1tb,2077-11.html

http://www.tomshardware.com/reviews/ssd-performance-power,2279-4.html