tony - everything I've read is that FB is discouraging ANY type of marketing message in the cover. Here's what's listed in the Pages TOS:
All covers are public. This means that anyone who visits your Page will be able to see your cover. Covers can't be deceptive, misleading, or infringe on anyone else's copyright. You may not encourage people to upload your cover to their personal timelines.
Covers may not include:
i. images with more than 20% text;
ii. price or purchase information, such as "40% off" or "Download it on socialmusic.com";
iii. contact information such as a website address, email, mailing address, or information that should go in your Page's "About" section;
iv. references to Facebook features or actions, such as "Like" or "Share" or an arrow pointing from the cover photo to any of these features; or
v. calls to action, such as "Get it now" or "Tell your friends."
Having spent the last three years dealing with Google (they suspended my local business page for violating one of 25 quality guidelines but wouldn't say which), I'm real reluctant to put a toe over the line with another giant, faceless company.