The driver is: 8.552.0.0 (10/28/08).My guess it is a bug in the driver since other drivers have no issues with it at all (that I know of). And it wouldn't make sense either, to restrict the programmer to only use const integers to index language-specific arrays.
I have gl_FragData[n] just for MRT purposes. Good to know about these issues though. Why is it an issue? Just to be annoying?