This is one article in the larger Troubleshooting SharePoint Alerts guide

Problem

When you try to use the Approval workflow in SharePoint Server 2007, e-mail message are sent in plain text format instead of in HTML format.

Fixed In

• KB 937906 – SharePoint Server 2007 hotfix package: May 25, 2007

This hotfix is believed to have been included in

• WSS3 SP1 (February 19th 2008)

• WSS3 SP2 (April 24th 2009)