The PGF system has been almost entirely rewritten, resulting in a much more powerful system than what we had in version 0.65. Most importantly, there is now a special frontend layer that introduces a simple, powerful syntax.
The new version works well enough and should be completely compatible with the old one. However, since there may still be syntax changes with the new frontend and since this version is not yet very well tested, this is an alpha release only. So, feel free to try it, but if you need a stable system, wait for version 1.00 of pgf.
There is also a beamer update (version 3.05) for use with the new pgf system. However, there are much less changes in beamer than in the pgf system.