You could do something with the site theme and the header and footer
We did exactly this 2 weeks ago after a major upgrade. We put a right-aligned text banner in the header in a nice contrasting color, linked to an internal "Release notes" page. Next time I'll probably do a transparent image so that I can match the font to our header image.
Another option considered was replacing the normal header image with an animated GIF, say the typical image morphing into a "New, Improved!" type of label, and make the image redirect to a release page, vice the main space. If you have graphics gurus in your organization, I'd recommend that.