will be gone when you need it.
Your letter should be specific. It should refer to the version of the schedule that shows the missed milestone, the date that was missed, and the actual impact. For example, "The project schedule dated September 15, 2003, shows your approval being required by December 12. To date, your failure to provide requirement approval has resulted in a two-week delay in starting work on the product design. As the product design is on the project critical path, this has resulted in a minimum of a two-week delay to the project end date."
Your letter should be timely. After the fact, it is nearly useless to try and claim schedule delays caused by others. Communication regarding schedule delays should be sent no more than two weeks from the start of the delay, sooner if the schedule is tighter.
Your claim should be reasonable. Delays will only be accepted if the missed milestone can be shown to be on the project's critical path. Of course, any missed milestone will eventually turn into the critical path. Failure to approve an insignificant portion of the project by the declared date will not typically result in an overall slide in the project.
As powerful as this communication is at the beginning of a project, it is actually most useful at the end of a project, particularly when you are in a contractual relationship with your customer. If you are delivering the project months late, you had better have a plan to find your way out of the forest. Night is coming, and the forest is full of wolves.