The Register and others are reporting a new zero-day exploit that targets Java Runtime 1.7. Recommended actions include disabling all browser based Java plugins. Dropping back to JRE 1.6 is not recommended as that may open other vulnerabilities.
Link to original Malware Intelligence article.
Link to The Register article.
Link to Sophos article
/Disclaimer: I have no way to verify the provenance or significance of the reports.