This report is a very interesting read. It discusses recent trends and data collected around viruses, malware, exploits, etc. Some of the conclusions:
"Disclosed vulnerabilities for 2006 rose 41 percent over the previous year, continuing an upward trend in new vulnerability disclosures. More vulnerabilities were disclosed in the second half of 2006 than in any single year from 2000 to 2004."
"A much larger percentage of vulnerabilities were "complex to exploit" than in previous years, supporting the observation that the security researcher industry is maturing and utilizing better tools and techniques to find more complex issues."
"Application vulnerabilities continued to grow relative to operating system vulnerabilities as a percentage of all disclosures during 2006, supporting the observation that security vulnerability researchers may be focusing more on applications than in the past."
Here's a link to the report.