Having always been somewhat of a daydreamer I often find myself in such situations so I was able to respond without hesitating. I've formed two standard responses to combat such wilfully thoughtless intrusion, "Yeah, that sounds good" or "Can we foresee any problems with pursuing that?". Now the first one is a bit of a gamble, you might be agreeing to something that you really don't want to agree to. You might have just agreed to actually, you know, do some work. Dreadful. However the second one is not fool proof, although at first glance it might appear to be infallible. Answering a question with a question is an excellent way to divert attention away in situations where you're not really listening or simply just don't give a crap. The problem is that this "foresee problems" bomb might just be completely irrelevant to the question, and then you're going to look like a dick. A dick that wasn't listening and tried to pretend he was. Saying "Yeah, that sounds good" when it's not relevant is relatively easy to gloss over, but if you demand to know the likelihood of any ill consequences when someone has just asked you if you want a coffee, you're going to look a bit of a spanner.
When you're in a meeting discussing the possibility of implementing major new modules, sometimes the stakes are too high to gamble, but this was a relatively informal one so I took a risk. I plumped for "Yeah, that sounds good" and was delighted to hear the response "Of course that's all just stuff for the future". Well, duh...