My co-worker fixed this issue. The zimlet attempts to use 'WhatId' to add a contact which requires the use of 'WhoId'. We corrected this issue in our local copy and I'll attach it here for use by anyone who would like to use it.
This copy also adds two other fixes which our sales staff asked for:
1) it allows scrolling within the zimlet by constraining the <div> to a 225px size as well as setting the overflow to 'auto'. Yes we have encountered long lists which overflow off the screen.
2) it doesn't check any boxes by default, which our sales staff found annoying and time consuming to uncheck.
I'm not sure if you'll find these other fixes helpful, but they are fairly easy to revert.
Hopefully some of these changes can filter into the main sforce zimlet.
EDIT:
I believe we need to fill out a Contribution Agreement for this to be legit. Hopefully this suffices:
Here is our Contribution Agreement:
Signing As: "Company"
Full Name: Elliott Johnson
Date: 3/12/2007
Email:
elliott@iparadigms.com
Zimbra Community ID: "elliott"
Address: 1624 Franklin Street, 7th Floor
Oakland, CA 94612
Company: iParadigms LLC
Your Title: System Administrator
Signature: Elliott Johnson