I think a good chance why this is true (that you *hear* about gifted kids either being early talkers or late talkers ) is because it is noteworthy. I think, when reading about people's experiences, you tend to see a lot of extremes. Not because there is necessarily a lot of extreme behavior, but because when there is it is commented upon.
If you just look through the threads on MDC, you'd assume that there are tons and tons of kids who potty train at 12 months or potty train at 5 years. I think both of those situations are actually pretty rare, but when they happen it is more likely the parent would mention it. I know I generally post asking for input when something is out of the ordinary. I'm not going to come on and say, "Guess what my child had their first word at 12 months, walked at 13 months, potty trained at 2.5, etc..." because it is so easy to find support/input on those occurrences IRL. Its when your child is doing something far outside of the norm (either early or late) that you are more likely to talk/write/question about it.
I would expect that most gifted children would be *on average* early talkers--- simply because gifted children, by definition, are advanced compared to their same aged peers. That said, there is a HUGE range of "normal" (using the term normal statistically here) so you are going to see gifted children who do anything quite early and quite late.
That's true... like my first DD wasn't toilet trained until she was 4 or 5 even though we spent years trying to get her to be toilet trained finally, and I asked my doctor if this was bad, because you know that's an awfully long time to not be completely toilet trained.
Oh, andi'm sorry if this is a little off subject, but at my DDs' school, the sixth grade were learning about child prodigies and comparing them to early man (I think...), and there was this one about a girl who became a college professor at age 24, and one of the things that jumped out at me was that she was reading Charlotte's Web by age eight months, and I thought, "That's crazy! How can that happen!?!?" but it also made me wonder how her parents knew that. that must have meant she learned to talk at age 2 months or something- really abnormal, in my opinion.