This techrepublic article provides a useful summary of IE7 security features:
http://articles.techrepublic.com.com/5100-1009_11-6130844.html
Firefox 2 is also emphasizing security features:
http://news.com.com/With+Firefox+2,+Mozilla+touts+security+and+speed/2100-1032_3-6129141.html
http://developer.mozilla.org/en/docs/Security_in_Firefox_2
http://news.com.com/Security+Bites+Podcast+Firefox+2+beats+IE+7/2324-12640_3-6130309.html