Javascript in emails: Bad idea? (Updated)

I asked on Twitter on October 26th if people thought it was a bad idea to include Javascript in email. The response was universally that it was a "really bad idea." As Unica's Len Shneyder put it, "Terrible idea! [It's a] potential security hole and like a .44 magnum to your toe." He goes on to point out that lots of places will block your mail based on javascript content. I've run into this myself; spam filterers find javascript in email to be a security risk.