Is it possible to define template files for the context-menu?
There is no equivalent user or system wide feature in Mac OS X 10.6.

However, many applications support Stationery Pad documents. These are documents marked as read only and when opened create a new untitled document using the original document's content.

The Stationery Pad option is available in the Finder > Get Info panel:



tuts+ has a tutorial explaining how Stationery Pads can be created and used.