I'll have to look more closely at the 5G skins as I hadn't noticed the technique in use.
Actually a border can be done pretty easily (I've even tried it already). The only problem is when there's no album art, if you have text that displays over where the border images are positioned there's a noticable gap. I just threw in a red border on the right of the album art in my simulator for this example:
Looks fine with album art:
But when there's no album art, the invisible image shows a gap over the text:
This can be avoided when using scrolling margins to avoid overlapping the text over the border, but you lose the use of the entire line. I'm sure there's a way to still make it look good creatively, but I haven't racked my brain trying to do so.