A light client has to trust other nodes to not collude to tell it lies, but doesn't have to trust a single node; it can request reports from several random nodes and compare notes. If node A says XXX = 5 BTC and node B says XXX = 10 BTC, ask nodes C, D, E & F to see what they say (by default, the bitcoin client talks to at least 8 randomly chosen peers). If all of the others agree with A, node B is lying to you, and you can safely ignore it forever.
As for bootstrapping a fresh full node, a full blockchain isn't actually required. The reference client is built with both a zero trust methodology and a high degree of paranoia, both great places to start for such a project. However, a full node can be altered to start it's own bootstrapping from 1) an internally hardwired & pre-pruned copy of the blockchain, which it trusts automaticly because it's part of it's own code, 2) from the most recent 'checkpoint' encoded into it's own code (search for the term on the forum) in much the same way that the genesis block is encoded into the clients' codebase now, or both. It's neither necessary, nor particularly helpful, if every new client has to start from the genesis block; eventually clients that start with an internally checkpointed block number from within the past year or so will be much more common. A client that uses both methods can entirely skip years of pruned transactions and hashwork, and still mine against the resulting pruned blockchain.
Non-colluding, so a light client is in some ways like a Ripple that actually works?
