The latest build of iPhone OS 2.0, 5a274d, includes a a preference panel that allows users to control settings for applications with push capabilities. The primary intent of this function: allow users to conserve battery life by turning push capabilities off for applications like Mail, which supports push for Exchange and Yahoo! Mail servers.
As we've noted before, push (which allows the iPhone to notify you of new mail as soon as it hits your inbox) or frequently checked email can slurp iPhone battery. Under current, public iPhone OS builds (including 1.1.4), users with psuh Yahoo! accounts can navigate to Settings > Mail then select the Yahoo! account and choose "Advanced" and select or deselect the "Use Push Mail" setting.
The new "Fetch New Data" preference in iPhone OS 2.0, 5a274d presumably controls push capabilities for both Exchange and Yahoo! Mail accounts, as well as future Apple or third-party applications that support push.