The TC-PS30 series is Panasonic's least expensive plasma TV for 2011 to feature 1080p resolution. The main features-related difference between it and the TC-PST30 series is that the S30 lacks the ability to display 3D TV.
We also expect there to be some differences between the picture quality of the two, because the ST30 uses Panasonic's so-called Infinite Black 2 panel, whereas the S30 does not. Because of this difference, we suspect that the S30 might have worse black-level performance than the ST30, but until we test both, we won't know for sure.
Both models have similar styling and feature sets otherwise, including Panasonic's new Viera Connect suite.
For more information, check out our writeup of the Panasonic TC-PST30 series and our look at Panasonic's 2011 plasma line..