The stuff on psionics in the SRD (which does not include githyanki, githzerai, mind flayers, and a lot of the "flavor" in the Psi Handbook) is in fact OGC.
So, yes. Psionics - at least that portion contained in the SRD - are OGC.
The ones in the draft srd at www.opengamingfoundation.org are superceded by the released official srd at the wizards site. All of it on the official site is OGC. It does not include mindflayers or Gith monsters, but it does have all the classes, powers, feats, and a lot of monsters.
I've heard but haven't confirmed that the prestige classes now give manifester levels instead of their own power point progressions.
Bruce Cordell wrote up alternate progressions for the prestige classes that increased manifester level instead of their own progressions. It was either in If Thoughts Could Kill or can be downloaded free from the archives at www.montecook.com, I can't remember which.
These are the only re-writes of the prestige classes I am aware of at this point. I'd be interested in finding out if there are other versions of them out there.