Use of links to alternate "GeoWeb" representations of HTML documents is a good and growing practice for adding geographic content to the web. For example:

<!-- context: http://example.com/places/638856.html -->
<link
  rel="alternate"
  type="application/vnd.google-earth.kml+xml"
  href="http://example.com/places/638856.kml"
  />

or

<!-- context: http://example.com/places/638856.html -->
<a 
  type="application/vnd.google-earth.kml+xml"
  href="http://example.com/places/638856.kml"
  >
  View in Google Earth (TM)
</a>

Another class of applications benefits from links to different, not alternate, geographic resources that describe where resources are using maps or markup from the geography domain. Search engines might use these links as a sign that the document may be spatially indexed. Browsers or plugins might fetch the link target for display in a map-let, or use the information in a search to find other virtually adjacent resources.

HTML example:

<!-- context: a document containing a news story -->
The flying saucer landed in a field outside <a rel="where" href="http://www.geonames.org/5577147/fort-collins.html">Fort Collins</a>.

Atom example, using an IRI form of the where relation type:

...
<entry>
...
<link 
  rel="http://example.com/relation/where"
  href="http://www.geonames.org/5577147/fort-collins.html
  />
...

See also: http://www.ietf.org/internet-drafts/draft-nottingham-http-link-header-05.txt (about link relation types in general) and http://tools.ietf.org/rfc/rfc4946.txt (one particular link relation RFC to emulate).