I've often thought about writing something similar myself. Here's a few things that I've considered for inclusion (in no particular order) ...

1 - Should support acknowledgement of message.

2 - Statistics - For each message sent, it should be possible to gather statistics on where it was displayed and acknowledged. This will help measure its effectiveness.

3 - Should support targeted by group messages (e.g. Everyone, Staff, Students)

4 - Should support hyperlinks to take user to more detailed information

5 - Should support 'silent' mode so as not to interrupt presentations. Perhaps it's possible to detect an app running full screen? It should also be possible to override this for super-critical messages.

6 - Command line interface so that automated messages can be sent

7 - Should not seize focus as this would interrupt the user

8 - Should support targeted messages by user environment (e.g. target thin client sessions only)

9 - Should not rely in having a presence in the tray although could still support it

10 - Should have low memory/cpu footprint to minimize impact on thin client

I was thinking of using AutoIt to develop this but am not happy with the memory footprint.