I haven't run across this before, so I thought I'd share.
Here's a simple command for looking up the current word in Dictionary.app:
osascript &>/dev/null \ -e 'with timeout of 30 seconds' \ -e 'tell application "Dictionary"' \ -e 'activate' \ -e 'end tell' \ -e 'end timeout' \ -e 'do shell script "open dict:///$TM_CURRENT_WORD"' &
We can't just do the shell script because, for whatever reason, it only works if Dictionary.app is already open. Without the "Timeout" argument applescript will not wait for Dictionary.app to open before requesting the definition.
I'm sure there are better ways to code this, but it works for me.
--oliver