The difference is not actually that clear but the exception seems to be that the E-PL1s now comes with a new battery, the BLS-S instead of the BLS-1 and an additional upgraded ISO range now reaching 6400 as opposed to 3200 on the E-PL1. That's about it.
Now sold with a new M.ZUIKO DIGITAL 14-42mm F3.5-5.6 II lens, it is ultra light too, weighing in at 454g, now that's officially the worlds lightest interchangeable camera, to date. Pretty amazing stuff by Olympus.
Also, you need to be aware that this model is only available in Japan and will not be made available elsewhere at present.