I use with pleasure SEO by Yoast, best plugin I think fr SEO (I tested a dozen)
if you happen to have your blog archives redirected to homepage
( like ...yoururl/blog/2011/02 redirect to /yoururl), like from the default WP archive widget:
Just uncheck the Robots page option of the plugin
"Disable the date-based archives"
I think it would be nice have a mention about this as well in the plugin page...