Ok, I've been giving a whirl on supporting Eclipse 4.2 on PyDev and everything seems to work as expected, but unfortunately, I still can't recommend the Eclipse 4.x series over the 3.x series because performance-wise Eclipse 4 is still not on par with Eclipse 3 (and for me the only gain was that floating docks are now better -- as for skinning, I don't really think it's much better for final users unless someone else already got to the point of doing a skin exactly as you'd want -- so I think that for now, Aptana Studio 3 skinning (themeing) is a better solution -- and targeted at Eclipse 3.x).
I have hopes that those issues will be addressed in a newer release of Eclipse 4, but the truth is that right now Eclipse 4 feels sluggish when compared to Eclipse 3 (even changing configurations on the Eclipse appearance preferences for having some performance gains).
So, if by any chance you've just gotten into PyDev and it doesn't seem as speedy as you'd like, please make sure you check it with Eclipse 3.8 (in the downloads page: http://download.eclipse.org/eclipse/downloads/ there's a link to Eclipse 3.x downloads available: http://download.eclipse.org/eclipse/downloads/eclipse3x.html).
Also, make sure that you have the latest version of java as it may make a difference.
Or, if you've gotten the Aptana Studio 3 download directly, you're already good to go -- as it's already based on Eclipse 3.x and has PyDev preinstalled (see http://pydev.org/download.html for links).