This script will apply user-entered text to the "Description" tag of each of the selected tracks. iTunes 5.0.1 or better.
(Note: In order to launch this PowerPC-era script on an Intel machine, it must first be opened and then saved using AppleScript Editor, which is located in your /Applications/Utilities/ folder. More information is here.)
Change video kind and video related tags of selected video tracks
Resample selected tracks' artwork to user-set size limit
Remove iTunes tracks disassociated from files
Set year of selected tracks to their release date's year (if available)
Re-locate files of selected tracks to new folder, preserve track meta-data
Perform search-and-replace on text in your choice of tags