Good idea, but the implementation needs to be fleshed out to prevent accidental issues.

I'll type this as far as I can without making it a Brainstorm proposal.

This would definitely need to be added not as something in the OS by default, but possibly as part of a "Downloadable Gestures" application.

This application would have two sections of configuration
- Gestures: set default gestures and teach new ones
- Applications: set the default/customized gestures to specific applications or system-wide

Common Gestures (for default)
- double-tap to zoom in/out
- half circle up/down for page up/down
- small "X" on a list item to delete an item
- wave (accelerometer senses device held in portrait mode while being shaken agressively) to send contact information to another recivable device
- tap then tap-drag right to bring up a context menu for an item (think copy/cut text or bring up menu for listed items)
- multi-touch for multiple selections (see post 13 in this thread): user taps on the outer left edge of a list element, and while finger is still on the screen, uses another finger to tap additional items for a multiselect; when left is lifted get the context menu to copy/cut/delete/dupe items.

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In other words, its all possible, but there's a lot of work to be done to ensure that it stays simple. Here's a start, if a developer is willing, there ya go for another application (man I wish I could code).