The authority of the site has given a brief explanation, “This afternoon a phishing attack was conducted in the forums and through sitemail. This attack created a fake istockphoto.com login screen, prompting users for a username & password, saved them to a malicious server, then redirected the user back to the iStockphoto main page.”

The site is taken down to prevent more vulnerabilities. Lets see if istockphoto will come up soon.