Yellow exclamation mark in references: What does it mean

In my c# project, i have references to c++/cli and native c++ projects under "references". (i need to have them there, in order to be able to use msbuild on my solution from the command line.). all of these projects have yellow exclamation marks overlayed to the "reference" symbol. (vs 2010). i can't find any explanation what these symbols are supposed to tell me.

any ideas ?

WM_THX
-thomas woelfer