Only when you try to apply modifiers is it limited to object mode. When you activate the modifier you can edit your objects and so on and the changes will be reflected as expected, but try to hit the apply button (after you are sure you’re done editing) it can only be applied in object mode.
In fact, turn on the modifier in edit mode (there will have to be a slight offset of the geometry from object center to get a mirror), do some editing however you want. In object mode the original object and the mirrored one will be one object. Once you’re done, you can hit apply in object mode, since this is the only place where modifiers can be applied. Now you have one object, but each piece of geometry is editable on its own in edit mode. Make sense? The theme here is that all modifiers are to be applied in object mode, but many can be manipulated in edit mode as well as object mode.
It is working in 2.53, so I think you’re having a workflow issue. Need any more help, just ask.