how to create a link field which validates to a set of nodes? Is there a way to set the Validation for a Link field so that authors are only allowed to link to a pre-determined certain set of nodes?

IOW: when authors add a link field to a Node I want them to be limited to only links that are on our site -and- which are webforms or other nodes. I do -not- want them to be able to freely link to other domains.

Ideally, I'd like to be able to limit this set of valid links to nodes of a certain content type, or with a certain tag.

Just to be clear: these are not site visitor form fields, these are the fields for -Authors-.
try the advanced link module:

Provides autocomplete widget for CCK Link field and additional
functionality over standard field:
Variants of URL filtering (external or internal).
Additional filter options by using URL patterns.
Autocomplete link field for internal mode - displays suggestions from existing url aliases and menu items
Option to allow users select url title from predefined list of values
Use cases
You want allow users specify link only to several sites (i.e. specify link only to facebook or twitter)
You want restrict users to specify only internal or external links
You want to use autocomplete suggestion when specifying internal links