Problem:

Exchange 2007 SP1 Will Not Support the Searching of Content Within Office 2007 Attachments in Messages Until the Respective Office 2007 Search Filters are Installed

Because Exchange 2007 SP1 does not include the search filters from the 2007 Microsoft Office release, Exchange 2007 SP1 cannot index Office 2007 attachments in mailboxes. Therefore, users who have mailboxes on Exchange 2007 SP1 Mailbox servers will not be able search their mailboxes for content within Office 2007 attachments in their messages. However, as soon as the Office 2007 search filters are available, installed, and integrated with your Exchange SP1 installations, this search functionality will be supported.

2007 Office System Converter: Microsoft Filter Pack
This download will install and register IFilters with the Windows Indexing Service. These IFilters are used by Microsoft Search products to index the contents of specific document formats. This Filter Pack includes IFilters for the following formats: .docx, .docm, .pptx, .pptm, .xlsx, .xlsm, .xlsb, .zip, .one, .vdx, .vsd, .vss, .vst, .vdx, .vsx, and .vtx.
http://www.microsoft.com/downloads/details.aspx?FamilyId=60C92A37-719C-4077-B5C6-CAC34F4227CC&displaylang=en

How to register Filter Pack IFilters with Exchange Server 2007
http://support.microsoft.com/default.aspx?scid=kb;en-us;944516

More information:
Exchange 2007 SP1 Release Notes
http://download.microsoft.com/download/5/e/6/5e672458-592a-44a2-b489-11cec19d3c82/RelNotes.htm