Someone emailed me today asking if it's possible to use AppleScript to make a certain file open with 
a given AppleScript droplet. It is indeed.

Since a droplet is just a special kind of application, the code is the same as for opening a file with
a specific app. It's just as simple as this:

set theFile to "Macintosh HD:Users:irate:Sites:some_file.html" as alias
set theApp to "Macintosh HD:Applications:Thistle Post" as alias
 
tell application "Finder" to open theFile using theApp

Hope someone finds that useful.