From what I understand, the 'solar()' and 'illuminate()' statements are only applicable to 'light' shaders though Aqsl will happily let you use them within 'surface' shaders (and possibly others) should the syntax be correct.
The problem is that a 'surface' shader using this call will crash Aqsis when invoked within a scene.
Though it could be argued that the shader author should know not to use this function, Aqsis shouldn't crash either.
Attached is a simple example (image, shader and RIB) which will crash approximately 35% through the render... when the offending 'surface' shader is executed.
This has been tested with Aqsis 1.2.0 (alpha 1) under both Windows 2000 SP4 and XP SP2.