How secure are various models of Bitcoin clients? 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".
One way to look at security is to identify the risks. For a bitcoin client there are at least :

1. Theft of private keys 

Mitigation:

1.1 Keys should be encrypted. 

1.2 Keys should not available to servers. 

1.3 Key generation should be done securely. 

2. Loss of private keys

2.1 You should be able to backup your wallet or private keys. 

3. Client software fails or is untrustworthy 

3.1 Software should be open source so that you can build it yourself and inspect the code. 

3.2 Software downloads should be signed by private key of author to prevent 'look-a-like' malicious software. 

3.3 Author should spend time building their reputation (even if pseudonymous).

3.4 The software environment the client runs in should be resistant to attack. 

MultiBit's scorecard

For MultiBit - which I am the lead developer on - the "Security Scorecard" is currently:

1.1 Currently no, planned for 0.3.0 release.

1.2 There is no server - purely local.

1.3 Keys are generated locally.

2.1 You can backup your wallet.  Key export and import is planned for 0.3.0 release.

3.1 Code is open source with an industry standard (Maven) build.

3.2 Currently no.

3.3 Author is a real public person - Jim Burton.

3.4 MultiBit uses a Java Virtual Machine - its security depends on the desktop machine not being rooted.