Just received an email notification along the lines of:
Subject: Salesforce.com API usage exceeded threshold of 25% This is an automated notification sent to you at your request by salesforce.com. Your organization has made 1251 API calls within the last 24 hours. This is 25% of your organization's 24-hour API call limit of 5000 calls. You will receive this notice once every 24 hours until your organization's API usage drops below 25% of your 24-hour call limit (1250 calls). For more detail on your organization's API usage, please review the API Usage Report under the Reports tab. If you wish to review or change your notifications, the settings can be found in Setup under Administration Setup > Monitoring > View API Usage Notifications.
The report can be found here (you may need to change from the na5 server):
Reports > Administrative Reports > API Usage Last 7 Days