STUFFIT  Per http://free.abracode.com/cmworkshop/macosx_10_6_snow_leopard_and_contextual_menu_plugins.html

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Contextual Menu Plugins in Mac OS 10.6 (Snow Leopard)

Snow Leopard does not support contextual menu plug-ins in 64 bit applications

Apple removed native support for contextual menu plug-ins in Mac OS 10.6 in 64 bit mode and replaced it with services items. Even if the contextual menu plug-in is 64 bit, the applications will not load it. Almost all, if not all Apple applications shipping with Snow Leopard run 64 bit by default, including Finder, so your contextual menu plugins will not work.  
(Version 14.0)

STUFFIT  ...a portion of my post was cut off...

The Services menu does not support the same "contextual behavior" (ie: presenting different commands depending on what files are selected). We are investigating alternate solutions.

MagicMenu, which provides the identical features as StuffIt Contextual Menu, works fine under Snow Leopard in both 32 and 64 bit modes.

Also, this comment was posted to the Expander product, but really only applies to the StuffIt Deluxe product.   
(Version 14.0)