Stowe Boyd
Subtags are an extension to the hashtag convention ('#sxsw') but adding a suffix ('.food') to indicate a subset of the root tag.
Subtags are a response to the tag meltdown at SxSW this year, when so many posts were tagged '#sxsw' that searching for the tag as a filtering approach just stopped working.
The idea is that subtagging is different that just adding multiple tags. For example,
"I can't say enough about the pairings at dinner last night #frenchlaundry.wine"
means that post is about the wine at the French Laundry restaurant, unequivocally. Where the two tags could mean something else altogether:
"I had so much fun in Napa yesterday #frenchlaundry #wine"
Where the wine and the French Laundry are incidentally related.
A great case can be made for subtags vis-a-vis conferences, where typically tags are preannounced (so-called 'beacon' tags). Consider a hypothetical conference called Twitter Con 2009, where a root tag might be suggested: #twc09. The organizers might suggest (or attendees might simply start using) obvious subtags, such as these:
@ev said some very smart things at his keynote #twc09.keynote
@gregarious I am headed out for an hour, but I will be back for the #twc09.party later on
Looks like some interesting tools debut today #twc09.launchpad
@chrismessina Are you attending the #twc09.camp on Sat?
It turns out that Twitter search 'works' for subtags already, although the result shows the root tags ('#sxsw') as a link back to the search for the root, and the suffix is not treated as a link. In principle, if Twitter were to make the suffix point back to the search for the subtag, it would be 'supported' in a limited way.
Other services, like clients and tag repositories, would have to be amended to support subtags.
Stowe Boyd, Reaching The Limits: Twitter In The Large And Subtags, /Message
Daniel Terdiman, At SXSW, attendees confront Twitter saturation, CNet Webware