That would be expected as far as I know - don't worry about it unless you are experiencing problems - Exchange 2k7 is good at making use of all the available memory in the box (especially in a CCR environment).
Exchange 2007 will utilize as much memory as it can leaving just enough for system resources/applications. If your server has 16GB, Exchange will use up to 14GB and leave 2GB for apps/system. Make sure your pagefile is memory + 10MB. So if you have 16GB of memory, make your page file 16.01GB.http://msexchangeteam.com/archive/2008/08/06/449484.aspx