Update 7/14/11: When contacting Microsoft Customer Support to obtain the Interim Update, you can reference KB 2581545 (please note that the article will be available at a later date on support.microsoft.com).
We have discovered an issue impacting some customers who have installed Exchange 2010 SP1 RU4 into their Exchange environment and as a result have removed SP1 RU4 from Download Center and recommend customers do not proceed with any planned deployments of SP1 RU4.
A small number of customers have reported when the Outlook client is used to move or copy a folder that subfolders and content for the moved folder are deleted. After investigation we have determined that the folder and item contents do not appear in the destination folder as expected but may be recovered from the Recoverable Items folder (what was previously known as Dumpster in older versions of Exchange) from the original folder. This behavior occurs due to a customer requested change in SP1 RU4 which allowed deleted Public Folders to be recovered. Outlook and Exchange are not correctly processing the folder move and copy operations causing the folder contents to appear to be deleted. OWA and Exchange Web Services clients are not affected by this change and process the folder move or copy actions correctly.
Read more at source: http://blogs.technet.com/b/exchange/archive/2011/07/13/exchange-2010-sp1-ru4-removed-from-download-center.aspx