On the Inventory Collection tab of the Software Inventory Client Agent Properties recently I have stumbled upon an issue where the New icon has been greyed out and disabled, not allowing me add any new rules defining the software inventory scope.
After some googling around, this behaviour turned out to be documented and by design. In Configuration Manager 2007, there is a maximum limit of 64 rules. I am not sure if it is applicable to Configuration Manager 2012 as well, but it is worth to keep this limit in mind. Sometimes you learn things which are well documented the hard way.
For more information, refer to the Configuring Software Inventory Rules article on Microsoft Technet – http://technet.microsoft.com/en-us/library/cc181215.aspx