On the design guidelines page http://www.campaignmonitor.com/design-guidelines/ in the help section, there's a paragraph about an automatic inline CSS tool:
Add inline styles
Some email clients will strip CSS out of the head, but leave it if the style block is (invalidly) in the body. Gmail goes further and will strip out all CSS from the head or body, except for inline styles.
So once you’ve completed your design, you will need to go through and add inline styles to your elements. Campaign Monitor has an automatic inline CSS tool built in which can save you a lot of time.
linking to this page here http://help.campaignmonitor.com/topic.aspx?t=109
Unfortunately, the link is broken. Is there such a tool? I've also tried searching the help and faqs section, as well as these forums.
This tool is great, not sure if it's the one reference on the help page:
http://code.dunae.ca/premailer.web/ also gets my vote - great free utility.
I came across it several months ago when I inherited an HTML email from a client with the CSS in the head. Now I always use it on my HTML emails prior to final testing.
Apologies for the dead link, will get that sorted now. The Premailer tool is great, and we have a similar service built right into the software. Just look for the inline CSS option when you import your campaign content. You can see this in action here: