In Mac OS X 10.5 and earlier, clicking and holding on an item in the Dock would bring up a contextual menu for that Dock item. In Mac OS X 10.6, popularly known as Snow Leopard, unpopularly known as Leopard Service Pack 1 or “What am I supposed to do with my Power Mac G5 Quad?”, this behavior has changed. One of the new non-Exchange non-features in Snow Leopard is Dock Exposé. Clicking and holding on an application icon in the Snow Leopard Dock invokes Exposé for that application. This is the same effect you see when pressing the F10 key in Tiger and later.
If for some bizarre reason you prefer the old behavior (you backward, reactionary, Obama-hating Luddite), it is possible to bring it back. This is a Lap Cat Software exclusive — you heard it here first, folks! Launch the Terminal application and enter the following:
defaults write com.apple.Dock show-expose-menus -bool no; killall Dock
You’re welcome. Remember me fondly in your will.