That's your problem, the old poof.pdf is made as one image, the image is twice as wide as the poof though. on the left is the actual poof images on a black background and on the right is the part of the file 10.1 used as a mask to 'cut out' the poof for display.
Changed in 10.2 is now the dock uses a PNG, cool thing about that is, that there is no need for the extra wide image because you just draw the poof on a clear background and OSX 10.2 does the rest.
In order to convert old PDF's to new PNG's you need a graphics program.
Photoshop instructions:
Open the PDF and select the right half of the file.
Copy
Select the left portion and crop the file down so only the left portion remains.
create a new Channel in the channels palette.
Paste in the peice you copied (the mask)
now command click the new channel to load the selection.
Go back to the layer your poof image is on (make sure it's a layer and not a Background.
Hit Delete.
Save as PNG
Replace
Enjoy
PRSop
)