I think a good solution would be to have a "Start Mining" button on sites, so users wouldn't be wondering why they're at 100% CPU.
Currently CPU mining isn't worth it because of the inefficiency.
CPU mining (even if this were compiled code) is definitely inefficient. We're eager to see what advances in HTML5 can bring to the table, namely web-based CL.
Having a "Start Mining" button is a good idea. We've also considered a small embedded real-time chart of browser hashing speed, or something else to let the client know what their browser is up to.