Running through my head a scenario where an entity that wanted to gain by destroying the network could have the resources to manipulate difficulty and price simultaneously, albeit temporarily. There are three cases I can see;

i) drives difficulty and price higher together
ii) locks difficulty and price in tandem (presumably some kind of feedback mechanism)
iii) drive price lower and difficulty higher

They would achieve this through expending resources in both market activity and mining activity.
Case i) trivial, I don't see this as anything other than increased participation so no weakness there.

Case ii) interesting proposition but not sure if this is actually an attack or what would be the outcome, positive or negative.

Case iii) is the one to worry about I think. They would have to first buy up or otherwise accumulate BTC resources or have access to short-selling futures mechanisms as well as have access to significant mining resources that could be bought on-line in a co-ordinated manner. Then through concerted market selling and futures selling depressing the price and simultaneously they drive mining difficulty high by increasing mining to the max they create a dislocation in the natural pricing signal.
Net effect being that honest miners begin dropping off-line as they become uneconomic due to the pricing/difficulty dislocation. I guess the question to answer here is, how long could they keep it up and is it possible to sustain it sufficiently to destroy all value in bitcoin?

Probably should have spelled it out but I was discounting the >50% attack since that is well described elsewhere. Using the word "manipulate" to imply that the hostile had some resources to influence difficulty but not over-whelming, i.e. dominant resources.

Anyway, how big does the network have to be to be safe from the >50%?

Are we there yet?

No, nothing in this thread is right.

Price manipulation is possible right now because volumes are low, but it will cost you more than it costs everyone else, so you'll lose.

The scenarios involving technical manipulation are entirely founded on misconceptions.  The network really doesn't work the way you imagine it does.  Someone would need several orders of magnitude more computing power than the rest of the world combined to pull off a block chain manipulation, and it would gain them very, very little.