There is a difference between, and a need for, both a program manager and a project manager for data warehousing efforts. Neither is necessarily more important but when one person tries to do both jobs (at least for large projects), usually the program aspects get neglected.
The program manager, usually titled a Director, is responsible for ensuring support for the data warehouse at the highest levels of the organization, keeping the business areas involved and keeping the data warehouse efforts focused on ROI, measuring that ROI and rolling out the PR program for the data warehouse.
The efforts of the remaining members of the data warehouse team need to be focused on that which will generate high ROI and be supported by the organization. A strong link needs to be forged between data warehouse efforts and their effect on income and expenses.
There are many choices in terms of direction for data warehouse efforts. Each iteration brings its own choices of source systems, users and their applications, business rules and subject areas as well as technology and architecture investments. Tradeoffs based on business need, the ability to deliver and available budget are continually needed and will be critical to the success of the overall program. Inattention to these activities due to focus on the current iteration's deliverable is perilous to the program.
These are the roles of the program manager.
The project manager is managing the daily tasks, resources and deliverables of one of the data warehouse iterations. There could be multiple of these project managers, each working on different iterations, for a program.
For more information, check out searchCRM's Best Web Links on Data Warehousing.