Priorities tend to change frequently, so when you are asked whether you can do something, just say yes. It doesn't matter if you think it might be hard or nigh impossible, just say yes. It usually turns out that you can, in fact, accomplish said task or that the requirements mutate into a form that makes life easier. In rare cases where something ends up being overly difficult, it's easier to appologize later. Again, priorities will have changed and most likely what you're working on isn't the thing the boss cares about.