But maybe if you switched the iconography/expectation: use a 'person' icon, instead of the lock, for vanilla SSL. It would indicate you have verified the server's identity -- which is not nothing -- but you can reserve the lock icon for forward-secure servers.
We've got a question of granularity [2] and rather than make a professional value judgment in the average user's best interests [3], the proposal is trying to represent the entire scale and then expect the user to educate themselves about what the scale means, compare that to their environment needs, etc.
[1] How long ago was it, we were simply railing against insufficient key sizes or improper certificate validation? Anyone who 'learned' up to that day's wisdom would have to continually update their knowledge along with the rest of us to be able to make informed decisions on which colors were 'safe' for which uses.
[2] 'how secure' on a scale from 'unsecure' to 'SSL' to 'perfect forward secrecy'
[3] drawing a line on the scale, to the left of which is not secure and to the right of which is secure. And, when 'less-secure' options prevent valuable information that isn't necessarily 'security' (e.g. identity validation) using a clear method of distinguishing that.
I'd support three stages: 1) no encryption, 2) some/partial encryption, 3) "perfect" encryption.
For the partial encryption, the browser would provide an explanation of exactly how it could be compromised. For the case where the encrypted version does not have perfect forward secrecy, it could explain that if the server is compromised, this session can also be decrypted.
I don't think browsers should cover the banks' asses here.
Edit (since I can't reply deeper): I thought you were being sarcastic since I consider a gradient to be a really horrible idea. I'd want something where the browser tells me something. I don't want to wonder whether the new browser version changed the gradients slightly or a site improved its encryption strategy. Clear symbols and language is a must.