I can create rounded corners in many clients with
style=\"-moz-border-radius: 15px; border-radius: 15px;\"
However this CSS does not create
http://www.campaignmonitor.com/css/ displays all css supported in major email clients. Rounded borders didn't even make it to the list. On the page there is a link to a full PDF, which notes that border radius is ONLY supported in Thunderbird 2
please note that you have to embed the styles in the html elements to support a broad range of email clients
You can find the full list of supported HTML elements and CSS styles in the following series of articles:
Word 2007 HTML and CSS Rendering Capabilities in Outlook 2007 (Part 1 of 2)
Word 2007 HTML and CSS Rendering Capabilities in Outlook 2007 (Part 2 of 2)