Well there's seems to be some interesting changes...
All the promo art seems to suggest that Tieflings will be a PC races in the new PHB, which is about time, since Tieflings are one of the most apparent lasting legacies of PS... With some suggestion for more accessible Githyanki PCs.
Psionics as it is, seems to get some more core magic use. It's possible that magic may use power points and augmentations, and that arcane, divine and psionic are now just window-dressing for the powers.
They'll be using a bunch of the stuff for Tome of Battle, so fighters and other formerly non-spellcasting classes will gets spells/ martial arts maneuvers.
And levels go up to 30.
Whether or not these changes are beneficial to a PS campaign, that remains to be seen. But a lot of people on EN World want a sourcebook on Sigil, so WotC is probably considering it if they're listening to feedback.
What I want to know is how different the core mechanics and rules are. And yay 30 level base classes (so now there will, over 9th lv. spells-maybe. yay)