In the interest of helping out someone else, I was able to quickly fix the non-functioning cruise control in my 2003 S2000. It turns out that the plastic bushing/plug on the clutch pedal assembly that contacts the clutch interlock switch had broken. If this switch is not depressed, the cruise control will not work. I'll now replace the brake interlock switch plug/bushing just to be safe. It's even more important for the brake interlock switch to properly operate since that's what operates your brake lights. FWIW, I've seen this same thing happen in Miatas.

 

Not enough people post resolves. Thanks.