Google posted one of the Matt Cutts Q&A videos today, where he talks about the effects of site redesigns with redirects on search rankings. Here's the specific question as it was posed to Matt:
I'm changing the platform of my blog. All old URLs will redirect to new ones. But, since the HTML code and layout of the pages are different, do you lose search engine rankings?
"Well, search engine rankings can change when the page changes itself," Cutts responds. "If you're doing the 301s correctly - a permanent redirect from the old site to the new site, and if you're doing it at a page level - so from the old page to the new page - you should be in relatively good shape, but it's not just incoming links."
"It's also the content of the page itself," he continues. "So if you had a really good layout with a really clean design, where all the text was really easily indexed, and you move to something that was a lot more confusing, and maybe the text wasn't as easy for us to extract, that could change your search rankings for the downside, or for the negative."
"In general, we're relatively good about changing layouts and still being able to discern what that page is about, but here's one test that you could do: as long as you haven't done the transition yourself, if you can try making a few tests, where you can take the layout of the new page or the new site, and see if you can apply it in some very simple ways to the old site, then that's a way to isolate those, because it's just like any scientific experiment," he says. "If you do two things at once, and your rankings go down, you can't decouple what caused it. Whereas if you can change just the layout - even if it's only on a few pages, to try out and see whether your rankings change with that, then you'll know - was it more likely to be because of the redirects or because I was changing my HTML layout."
In terms of layouts, you may also do well to consider the role design plays in how Google determines quality content. Would people be comfortable giving your site their credit card info? Design can play a big role in this. Another question on Google's list of "questions that one could use to assess the 'quality' of a page or an article," is "Are the pages produced with great care and attention to detail vs. less attention to detail?" Then there's the whole load time factor. Google does count page speed as a ranking signal.