Robert_Charlton - 9:32 am on Feb 26, 2011 (gmt 0)
There was a time last July-Aug when there was a lot of discussion in Google SEO News about PR sculpting, and I made several long posts on these threads just to try to cover all bases. Not sure how well I did, but moments in these discussions might provide food for thought....
Robots.txt vs. meta robots noindex?
PR Sculpting Doesn't Work and Internal NoFollow Can Harm Your Site
Essentially, robots.txt would be sending your PR (and associated inbound link benefits) down a black hole... throwing it away. I'd perhaps use it to block a directory of search results, an https subdomain, or a directory of known dupe content on a site so large you need to conserve crawl budget, where you're not draining off link juice from too many pages.
Otherwise, if you must block a page, I think meta robots noindex,follow is the way to go.
I feel that theming is something that should be determined by your nav structure.