Inoperative cruise control due to the addition of the lower mesh grill was broadcast when the XLR-V arrived in '07. Since then, owners with base XLR's have installed lower grills with no detrimental effects. It is possible that during XLR-V testing there may have been issues that persuaded GM to opt out due to perceived safety/lawsuit issues, but we'll never know for sure. The bottom line is, there is no guarantee you'll lose your cruise control function with an aftermarket installation. Others who have done this can chime in and describe their results, or you can read their posts.
Just like your microwave door, the signals emitted by the device are constrained by the size of the little holes you see through the glass door or distance of the mesh in the grill. The wavelength of the frequency used in the XLR's radar is a little wider than a car lane (so as not to react to trees, etc.) Apparently, the size of the grill's mesh allows enough of the transmitted signal and bounced return signals to get through to the receiver.
There are two types of lower mesh grills -those (like the E&G) that fit over the plastic/air intake grill between the smoked plastic bezels, (the passenger/right side bezel is just for looks) and the official GM gill which spans the entire lower fascia. I haven't heard of either affecting the operation of the cruise control.
CC