Because Ace of Spades isn't a mechanic, I'd recommend starting with the basics, and using a paper clip or jumper wire to check for error codes, as documented in the techinfo section, or having someone scan it for codes.
If any codes are stored, that'd be the starting point for troubleshooting. The SES light will also turn on without setting a code if the engine is simply idling at too low of an RPM value, but unless there are obvious problems such as a new puddle under the car or something smoking, I really think the first thing to do is to check for error codes.
'99 Bonneville SLE, white, 112,000 miles
'02 Park Avenue Ultra, white diamond pearl, 88,000 miles