I wish people were more comfortable saying "I don't know but I can do some research and get back to you later." Often times that's where the conversation about time estimates ends because they don't ask you again unless it's actually important. And if they do ask you, now you have a better answer (assuming you actually looked into the problem you're trying to solve).

The other problem with time estimates that I find is that even though I might know for sure that a certain feature will take only a day or two, I can't tell when it'll be done because I usually have other things to work on as well.