The biggest changes are that Malicious File Execution and Information Leakage/Improper Error Handling are dropping off the list for 2010. In their place, Security Misconfiguration and Unvalidated Redirects/Forwards are being added. Some other items are shifting around. The chart below sums up the changes very nicely.
The release candidate of the new Top Ten is now available for download as a PDF document. OWASP is requesting feedback on anything and everything until December 31, 2009. I've not yet read the document in detail. At first glance, I wonder about the naming conventions. For example, is "Injection" descriptive enough? Is "misconfiguration" a real word? Why is "Insecure Communications" changing to the more cryptic "Insufficient Transport Layer Security"? I guess now is the time to ask!