I was trying to use cast() today and appreciate the 'value' argument. However, it does not interact how I would expect with "..." in the formula. The specified value column is not excluded. I think if you just renamed the column before parsing the formula, it would work as expected. This is with reshape version 0.8.3. Might be ancient, sorry if this has been already resolved.
I'm pretty sure that's fixed in reshape2, which you should be using unless there's some feature of reshape that you depend on.