|XP is just a number|
Just as an add-on to this scope-creap discussion ...
I think etcshadow (and tye too) you are right, if you have done all your analysis right, and have been in this business for long enough, you should surely be able too see the scope creap coming. This in no way implies anyone is smarter than the other, just that you have deep battle scars :)
Now one point which has not been touched on here is the politics of scope creep. I work for a small consultancy (we are down to 3 full time and a number of regular freelancers), and we do the majority of our work for one client, with whom we have had a relationship with for almost 10 years (predating the formation of the company actually). Our relationship with them is more along the lines of a partnership, rather than a contractor-contractee relationship. This basically implies much more give and take, since we want long term, not short term business from them.
I have found many times when small-to-medium-small scope creep comes up early in the project (after the coding has started, but before the "point of no return"). It is politically best to let it happen, all the while making sure they know that it is scope creep, and that you are willing to let it happen. The reason I say this is two-fold.
And of course, if your client is not being reasonable, and are pushing you to "just do it". They are clearly trying to take advantage of you, and you should act appropriately. And no matter how many times they bought you a beer when you were hanging out after work, or how many times you compared stories about your kids over lunch, when it comes to business, they are not and cannot be your friends. Business is business, and that is just how it has to be. If they try to drag friendship into it, then be wary.
* - I spent many a year working in the advertsing business where back-stabbing politics is just how things work, and I absolutely despised it. But, politics are there, they cannot and should not be ignored (even as a contractor, they may not directly affect you, but you need to be aware of their impact on your work).