IMHO, the metal casing of the MBP is surprisingly durable. My powerbook G4, after 5 years, looked almost like new when I sold it (only a very few cosmetic surface scuffs). My current MBP, a little over 2 years old, still looks like new too. Unless you intend on using it in some hazardous environments (prone to drops, impacts, etc) then there shouldn't really be a reason for you to need a hard case. It's a laptop, not a piece of irreplaceable art (although I do admit that the MBP unibody design is to some degree a work of art).
I just use a neoprene case for transport.
Besides, the feel of the unibody casing is really great, why ruin that with a cheesy plastic shell?
Ruahrc