As many of you are well aware through recent Microsoft Blog postings, the Connect forum and the Symantec KB Article 16949 there is an issue with space not being reclaimed by the Exchange 2010 store when objects are deleted or truncated. This impacts many scenarios some of which are prevalent when archiving.
Before we get into too much detail here's the good news - Microsoft have fixed this issue and it's available on request. For the fix go to http://support.microsoft.com/kb/2621266 .
This is an interim update (IU) and will hopefully be made available in an upcoming Rollup Update (RU) for Exchange Server 2010 SP2. There are something's to note for the fix, once installed it won't magically reclaim all space, there are undefined deletion thresholds that have to been passed...