The LAST big mistake of cloud computing
... is to NOT have an exit strategy.
Cloud computing is just another fad, It won't last forever - and it's highly likely that one (or more) cloud offerings will go belly up, have major infrastructure problems, get hacked to a crisp or simply get bought up by the next evil empire who will realise that all their customers are locked in and will squeeze them till the pips squeak.
So, on the basis that at some point in the next 10 years, a lot of outfits that are currently rushing headlong ito cloud services will want out, and want out FAST, it's important that these organisations have a way of pulling out - and that they don't wait until it's too late, but design and negotiate a solution to let them say "goodbye" with the least pain, downtime and cost.
However, this does seem to be a factor that is entirely missing from most company's IT strategy, or long term planning, I can only assume that any exit strategy that IS being planned is the exit of the decision-makers who committed all their IT to a single basket - and then ran for the door, leaving someone else to pick up the mess.