jdMorgan - 11:45 pm on Aug 24, 2010 (gmt 0)
The answer depends entirely on what -in the larger view- you are trying to accomplish.
Is your goal to redirect the client from one URL to another, to to internally rewrite incoming requests for the subdomain URLs so that content is served from a subdirectory of the main domain's filespace?
If the latter, we'll need to know how the two hostnames are hosted, specifically whether they are on the same server, whether they share filespace, and how the subdomain is currently "mapped into" the main domain's filespace.
Without this information, the only answer I could give would be "With server-side code."