I think the idea was to save money and add capacity, as laptop drives are more expensive and lower capacity...
However I wouldn't have thought it'd take too long before some enterprising peripheral company brings out a small enclosure big enough for a 3.5" drive which connects to the end of the PS3 and plugs into the existing HDD data and power connectors...
Which I probably should've patented before posting.
Heh, a programmer at work said the same thing two days ago...
I can picture a device that acts like a base for when it's sitting vertically and inside the base is space for the 3.5HDD.