Don't get me wrong, I don't think the build up is an issue. I know the durability cycles they run on these engines. But at 100,000 it seems like it's something I should do just to do it.By 100,000 miles I think you will find quite a bit of build up. What is interesting though is that while many people are posting that they are observing the build up, they are not very often posting that they are having problems. So generally this seems to be pretty benign. My Ford mechanic did say that the more catastrophic possibility is where a large chunk of carbon breaks loose from a valve and then makes it's way to the turbine and wrecks the blades.
It seems that everyone is going to direct injection, even my wife's new econobox Nissan has a 1.6 liter GDI engine. It will be interesting to find out if these new engines have solved the problem.
Here are some links that were posted earlier:
go to minute 16:13 for this one
Also everything is DI now, and I mean everything, even Chrysler is moving to DI and they are the last to do ANYTHING. However dual port designs are coming (Audi has them in Europe), so this will be a "generational" issue.