What I imagine is a trusted centralized server (say run by Mt. Gox or some other exchange), that exposes a simple medium/low level API (let's say in JSON):
I know BitcoinJ does't store the whole blockchain, but it does store and update the headers. The method I seek will save me the hassle of managing any local data except the actual keys/addresses.
I would not have to give this transaction my private key, but rather use some sort of Offline Transaction equivalent. Is there something close to this? Is somebody building this? (If not, someone should!)