The Service Engine Soon light is different than the Check Engine light. My first forae into the two lights was with my 05 Corolla. The Maint Req light would blink a few times on start up and then go out then it stayed on. It's the oil change or the interval for a required maintenance item (spark plugs, etc) that the mfg loads into the computer so you don't have to actually think... and then you spend the $100/hr to have Nissan look at the light and do about $30 in an oil change or a hour of labor and $20 for spark plugs.
My advice, just take it to your local mechanic, they can check out the same light for about half the labor cost. You can also have your mechanic change the intervals.
It is definitely NOT a cat, sensor or other engine issue, that would throw the check engine light. I was told that by a Nissan dealership. I thought I was starting to have issues with my cat (sulpher smell, ended up being the gas I was using). He told me if the cat is starting to fail the Check Engine light would come on, no matter how insignificant the change.
Just a thought.