I find myself deleting mail a lot out of pure laziness - it's simply
easier to hit the 'delete' key than to drag and drop a message from my
inbox into the appropriate folder.

Is there a way, in Outlook (2003), to create a keyboard shortcut which
will move the currently selected message to a particular folder? I'd
like a single key shortcut I can press which will move the current
message to, say, my OldMail folder.

you want to reduce two mouse clicks to one key click ?

the normal way to do it is stand on message , click on move to folder
, click on folder to move to

No, I want to reduce:
 1) take hand off keyboard and put on mouse
 2) move mouse to correct location
 3) click
 4) move mouse to correct location
 5) click
 6) move hand back to keyboard

to:
 1) type one character

Since Outlook does not have such functionaliy build in the only way
you can do that is assign a keyboard macro to a function key

if i am not mistaken Ctrl+shift+V is move to folder in Outlook 

so the macro should execute Ctrl+shift+V+first letter of folder to move to + enter 

there are numerous freeware programs available on www.download.com
that allow you to define keyboard macros

If by any chance your function keys are already taken you could assign
the same macro to one of your mouse keys

That won't work, because my folders are nested, and are in an imap store.