Pixelize Transition:  Available as part of the PoP Effects pack.

Built for Mac OS-X   
Decomposes or constructs your clip from blocky pixels.

The Pixelize Transition pack for iMovie contains the following transitions: Before
 After

"DePixel In"             

Starts with a very blocky version of the clip and gradually assembles into the detail
No example available

"Pixelize Out"       

Reverse of DePixel. Finishes with blocky version
No example available

"Pixelize Crossover"

Transition from one clip to next through a blocky version .
No example available