If you check the http://getshopped.org forums there’s a ‘sticky’ topic in the How-to section about this. There is different advice throughout it all, but here’s what I did.
1. In Store-Settings-Admin I set the URL’s to https
2. Added the code found on the getshopped.org forum to shopping-cart-page.php to force it to SSL
3. Installed a plugin to force WordPress elements to be SSL (I used HTTPS for WordPress)
I have read that WordPress 3.0 does this for you, it detects if the page is https and then changes all urls to https, but I’m not sure on that. I’m running 3.0 and also installed HTTPS for WordPress to be sure. You’ll likely still have to do some searching to make sure other plugins are not pushing non-SSL files to the page and clean those up to prevent any “This page is not secure” error messages.