The Exchange Team released the second cumulative update for Exchange 2013 RTM today. More information can be read from the Exchange Team Blog here.
A brief summary of the major changes:
1.As promised, the number of databases has been increased from 50 to 100 for the Enterprise edition of Exchange 2013.
2. Providing Single sign-on when using FBA on OWA vdir's
3. A new service for HA, the DAG Management Service
4. Improvements to the responder throttling for Managed Availability
5. Update-ExchangeHelp cmdlet for updating the local Shell help
6. OWA search improvements in terms of functionality and results returned
7. Customizing and applying granular Malware filter rules with the -MalwareFilterRule cmdlet