Hello,
I don't know the answer to this, but here's my theory... I doubt SharePoint
would be able to search attachments, because it probably doesn't recognise
the embedded file contents as attachments. When the attachments are added to
your InfoPath form they are encoded using base-64 encoding and then added to
the Xml contents.
You would probably have to either strip the attachments out, or write an
IFilter that would recognise InfoPath forms and allow SharePoint to search
the attachments. Neither of those options are particularly thrilling
More technical info (InfoPath 2003 but I believe it is the same as 2007
format):
http://blogs.msdn.com/infopath/archive/2004/03/18/92221.aspx
Gavin.