There is a bug in how ReSharper handles controls that are declared inside of public properties that have the attribute. In the code behind, ReSharper highlights the control in red, even though it compiles just fine.
I have reported this problem once before in version 4.0 of ReSharper, and it seems the problem still exists in version 4.5. I am hopeful that since v4.5 is still under active development, that this issue can be addressed this time. We have several controls that make use of this functionality, and so several of my co-workers have uninstalled ReSharper because of this bug.
I have attached a sample solution that demonstrated this issue. Note that this problem occurs for properties of type ITemplate as well as PlaceHolder.