If you try and add a network share/UNC path to the 'Trusted Locations' in Office 2007/2010, it will give you an error ("The path you have entered cannot be used as a Trusted Location for security reasons.") if the path you are adding contains an IP address instead of a hostname, e.g. \\10.10.10.1\share. You receive this error even if you have checked the "Allow Trusted Locations on my network (not recommended)" box. Apparently you are only able to add UNC paths, e.g. \\hostname\share.
There appears to be a very simple workaround to this - simply open up Internet Options and add the IP address to your 'Trusted Sites', (file:IP address). Sites listed in your Trusted Sites seem to also be treated as Trusted Locations in Office.
UPDATE 17.06.2014
As mentioned in the comments, this is how to get to the Trusted Sites tab in Internet Options:
Open Internet Explorer --> Settings --> Internet Options --> Security --> Zone: Trusted Sites --> Sites
or
Open Control Panel --> Internet Options --> Security --> Zone: Trusted Sites --> Sites