I've noticed a couple of problems with Infinite Scroll's footer functionality.
1. The width of the footer is inconsistent between my installs of FireFox (18.0.1) versus Chrome (24.0.1312.56) and IE (9.0.8112.16421). It's a bit wider on FireFox. I thought it was my custom theme, but it shows up also on the example TwentyTwelve blog (http://twentytwelvedemo.wordpress.com/blog/)
2. I'd really like to have more control in styling the pop-up footer. I can target some things like the opacity with CSS, but I can't modify the content completely without messing with the plugin. I found the 'infinite_scroll_credit' filter and am using that to modify the right side, but I'd like to be able to touch the left side too.