I was recently lucky enough to attend a talk by Sourcefire CTO Martin Roesch (creator of Snort).  He mentioned the Cybertrust Breach report which is wotrh a read.

One of the interesting facts is that the time from an initial breach of a network to compromising data is in MINUTES 27% of the time and in HOURS 21% of the time.  This is very startling data.  What the report concludes as well as Mr. Roesch is that folks aren’t monitoring their logs.  Almost everyone is running IDS/IPS these days and generating thousands of events but no one is actually watching them.  I’ll post a review of Martin’s talk in the next few days.

In the meantime you can find the 2009 breach report here.