During iteration retrospectives on agile projects, one of the topics that comes up most often for us is that the product owner is (or product owners are) not available or engaged in the project at a day to day level.
It seems to be a common theme that customers are unwilling to "give up" the necessary amount of their product owner's time to the project, but instead have them answer questions via email, or during product demos only. This has the effect of increasing the length of the feedback cycle and making the project less effective.
Have you had to overcome this hurdle? How did you do it?