Apart from the security issues as noted by other users, I have a few issues with this script as written that others might wish to be aware of.

First, there is no need to enter a password within the script; the user would be prompted for password on first run, and it can be added to the Keychain from there forward, where it will not ask again (unless the Keychain itself requires unlocking).

Second, I know I'd be pretty irritated to be switched out of whatever program I'm running to Finder just to find a text dialog with someone bothering me while I work; such a shift very well may botch an operation I'm in the middle of; at minimum it will interrupt my train of thought and is basically just plain rude behavior. If you must use this kind of script, best to use 'tell current application' or derive the frontmost app and tell it to display the dialog, so that at least the user is not ripped from their current program.

Third, I know I'd want to smack the message sender for messing with my clipboard, especially if I just spent most of five minutes shift-clicking a very difficult set of elements in Flash or AI and had pasted them to the clipboard, only to find my clipboard now contains 'How you doin'?' While it's possible in some bizarre work flow of the author's imagination, there is a reason to paste that text back somewhere else, best to add a button in the dialog that *optionally* pastes the incoming message to clipboard.

Finally, I'm with everyone else. For this script to work it has to be run on Macs which come with IM and email programs by default. Unless these apps have been removed or disabled for security reasons, they should be the carrier of intra-office communications, as they are already designed to accept messages and alert the user to said message without damaging the current work flow. If need be, an AppleScript can be run on the remote target to open iChat (or whatever) and set its status to available in order to accept the message.

Beyond that, yes, just use Apple Remote Desktop to send screen messages in a corporate environment.