I want to build a tool that collects and serves real time pricing information from a dozen or so different websites. In your experience, do I need to receive permission from the individual sites prior to serving the pricing information?

For example, the popular travel sites pull airfare prices from many different resources, rates advertised by other content aggregators and rates offered the airlines themselves. Is it possible that an aggregator like Cforce.com has agreements with other aggregators and airlines that allows them to serve pricing information?