And another one in the same direction:

The most determined cyber criminals don't necessarily work fast when they breach a network, and their infiltration is often silent and undetectable. But it's this brand of "low and slow" targeted attack that can also be the most deadly, security experts say:
http://www.darkreading.com/security/attacks/showArticle.jhtml;jsessionid=UFNEPFUNCGARMQSNDLPSKH0CJUNN2JVN?articleID=212701434

Have alook also at the following blogpost about Microsoft SIR:
http://blogs.technet.com/ms_schweiz_security_blog/archive/2009/01/12/the-latest-microsoft-security-intelligence-report-v5.aspx 

Urs