Exchange sustained engineering has released Exchange 2007 SP3 RU6 to the Microsoft download centre as update 2608656.
I'll leave you you to peruse the changes, but there are a couple in particular worth highlighting
2545080 Users in a source forest cannot view the free/busy information of mailboxes in a target forest when the cross-forest Availability service is configured between two Exchange Server 2007 forests
914533 The Microsoft Exchange Information Store service may stop responding on an Exchange Server 2007 server
2575360 A new feature is available to automatically stop the Microsoft Exchange Information Store service when a time-out is detected in an Exchange Server 2007 SP3 environment
The fix for the Availability web service is important to understand as it corrects the behaviour of the CAS when it chooses the URL to use for the target forest. This issue occurs because the Client Access server in the source forest uses internal URLs of the Availability service in the target forest.
When a user requests free/busy information, the Client Access server in the source forest uses the Autodiscover service to determine which URL of the Availability service in the target forest to query free/busy information. The Autodiscover service retrieves both the external URLs and the internal URLs that are configured for Exchange Web Services in the target forest. However, the source Client Access server only uses the internal URLs of the Availability service. Because the internal URLs of the target forest cannot be resolved by DNS, the request fails.
As always please evaluate the update in a lab prior to placing this into production!