According to the Owner's Manual the "SERVICE ENGINE SOON" idiot light is called the Malfunction Indicator Lamp (MIL) in Nissan-speak. Page 2-14 (2005) states the following:
"If the malfunction indicator lamp comes on
steady or blinks while the engine is running, it
may indicate a potential emission control and/or
The malfunction indicator lamp may also come
on steady if the fuel filler cap is loose or missing,
or if the vehicle runs out of fuel. Check to make
sure the fuel filler cap is installed and closed
tightly, and that the vehicle has at least three
gallons of fuel in the fuel tank.
After a few driving trips, the lamp should
turn off if no other potential emission control
system malfunction exists.
If this indicator lamp comes on continuously for
20 seconds and then blinks for 10 seconds
when the engine is not running, it indicates that
the vehicle is not ready for an emission control
system inspection/maintenance test. See
“Readiness for inspection/maintenance (I/M)
test” in the “9. Technical and consumer information”
section of this manual."
If I were you I would look at the simple things first. Check your gas cap to ensure it was put on properly (tight) during the last fill-up. A loose or missing gas cap is the #1 cause for triggering the MIL light.
My 2004 did the same thing about a month ago. I waited a week and brought it in for the alternator recall as well. They found 1 code ( throttle position sensor) 10 driving cycles ago. No subsequent codes - but the light stays on. They reset the code and may have done something else ( put the postioner back in learning mode? ). It has been fine since and they said if it occurs again they will replace the throttle sensor. Oh yeah, we're at 16K miles - btw.