Based on the manual and the Very Useful Feedback I've been getting from the Hive Mind, I'm gonna open the gas cap and re-seal it, then go check the tire pressure*, and see where that leaves me. EVERYTHING, including the manual, suggests that a) It's Nothing Urgent Or Critical and b) It May Just Go Away By Itself In A Few Days, esp. if I do those little things.
(If the light were blinking, of if it were the SERVICE light instead of "Service Engine Soon", that would be another matter.)
Consensus: If it's still shining merrily on Monday, then I'll worry.
Thanks to everyone for all the information and advice!
*My usual service station doesn't have an air hose, so I've been uncharacteristically remiss in keeping tabs on my tire pressure. Yes, this may have contributed to the blowout.