As there was some disagreement on security of various models of Bitcoin clients in comments to this answer, I'd like to address the problem.
How secure are various models of Bitcoin clients? Those would include the standard client, "lightweight client" (one keeping only block headers from old blocks), eWallets, client-server client (relying on one server for all information, but keeping a wallet separate), and the proposed Stratum client (relying on many servers for information).
Also, how does the security of such model rely on the person developing it (core Bitcoin team, reputable Bitcoin enthusiasts, people not known in the community)?
For example, "Stratum client offers security features X. If the server was created by a trusted party, the security concerns are Y, if the server was created by malevolent person, the security concerns are Z".