My 06 certainly has it, you just have to pull the shift lever back one more click. It brings the rpm's up a lot more quickly. I use it for faster passing as if you leave it in D it takes a lot longer for it to rev up.
In a 5-speed Insight, when driving in the mountains it's helpful to shift down to 3rd or 2nd to keep the engine revs up and save battery charge. Presumably this approach would work on the CVT models also...
My guess is that the L position sets the CVT all the way to the limit on the high ratio side, and that D and S use a range of ratios but the S position runs the engine faster.
Has anybody with a Civic experimented with aggressive use of S mode to avoid using the IMA system?