The tag definition site, tagal.us, could be made more useful if tags have a location. Trending tags for a given geographical area could be queried. The current least-effort approach is as follows. A tag is defined as free-form text. URLs in the text are converted to anchor tags. These URLs are added to a job queue. The queue is worked on as needed to scan the contents of the given URLs for location markup. If location markup is found, then the lat/long (address too?) is stored in the tagalus database along with that tag. Its a microformats based approach that takes no extra effort on the part of the tag creator other than including useful URLs in the definition.
For example, if a user is comfortable with Shizzow.com as a database of places, a description for the tag #UGNW might include this url http://www.shizzow.com/places/YIwYMj. Shizzow uses the "geo" microformat.
The HTML from shizzow's place page for the Urban Grind
911 NW 14th Ave.
<span class="country-name">United States</span>
<span class="latitude" title="45.52940800000000">45°31'46"N</span>
<span class="longitude" title="-122.68539100000000">122°41'7"W</span>
Regarding trending tags: the creation of tags come into tagalus directly. The use of tags do not. Unless tagalus gets a copy of the hallowed, proverbial "firehose" (total twitter stream copy), it might not be practical for tagalus to monitor tag usage. One way is for tagalus to hit search.twitter.com for every tag it knows about, using a frequency adjusted by tag hit count returned in the last search. Maybe trending is not part of tagalus's role.