These transitions are free addon if you have ShaderTFX.
One of the most popular transitions I hacked for XP were the Split Video Transitions. It is a pity that despite its advanced and powerful architecture Vista does not provide a simple mechanism to make these transitions. Well! actually there is a general inability to customize transitions in Vista Movie Maker.
However with ShaderTFX all those old tricks become possible again in Vista... And much more!!!
Below I have provided a set of split transitions made using the external texture capabilities of ShaderTFX. Basically you draw a black and white mask image for splitting image A (black area) and image B (white area). The shape can be arbitrary. Once the image is ready you need to specify it in the XML file like this:
<Transition name="RehanFX Split Center AB" iconid="0" guid="RehanFX Split cAB" ShaderModel="2">
<Param name="ShaderFilename" value="rehanfx_split.fx"/>
<Param name="Technique" value="SplitSimpleTransition"/>
<Param name="TextureFile" value="%ProgramFiles%\Movie Maker\Shared\AddOnTFX\Split\cab.png" />
Download this free addon for SahderTFX: rehanfx_split.exe
Also see this post on WMM Forums: