I need to create a "document" icon for an application on the mac, and I need a source "blank page" image to use for it. Is Apple OK with using their own blank page icon as the backing for your icon? If not, is there a free equivalent somewhere I can use?
Apple encourages you to use the system standard images that ship as part of Xcode.
You can see this clearly in section 2.E. of the Xcode 4.2 License agreement.
E. Use of System Provided Images. The system provided images, as defined in Apple's Human Interface Guidelines for the Mac OS and the iPhone OS, that are created and owned by Apple ("System Provided Images") are licensed to you solely for the purpose of developing Mac and iPhone applications respectively using the Developer Software.
I am not a lawyer, and there are further restrictions, but it is good form to stick with the provided templates unless you have a good reason and the time/skills/money to invent your own.