I was about to redo a Rails site today using AJAX so that the background could remain in place. Then I thought, "that would be exactly like using frames," and then I thought about SEO consequences and I dropped the idea entirely and took a look around the Web. It turns out that few sites -- except for Google itself, which has the link bar at the top -- are doing this. It doesn't look impossible to do, but it's not exactly easy. Aside from Ajaxing, you have to think about updating the query-string and having SEO-friendly links (that actually don't work, but rather make background requests via Javascript).
Will any of the new technologies -- HTML5, perhaps -- solve this problem and allow us to have a Web with background colors (and other static elements) that do not disappear momentarily between page refreshes?
On the other hand, why are few devs doing this with current technologies? Is it just not a big deal, too complicated to implement, or....?
