The <a href="..." rel="nofollow">...</a> may not do what you suggest. The authors' spec ( http://microformats.org/wiki/rel-nofollow ) states in the Abstract:

.... By adding rel="nofollow" to a hyperlink, a page indicates that the destination of that hyperlink SHOULD NOT be afforded any additional weight or ranking by user agents which perform link analysis upon web pages (e.g. search engines). Typical use cases include links created by 3rd party commenters on blogs, or links the author wishes to point to, but avoid endorsing.

WP puts it this way (apparently correctly):

The nofollow attribute value is not meant for blocking access to content, or for preventing content to be indexed by search engines.

It becomes clear, as the article continues, that the SE implementation is crucial:

While some (search engines) take it literally and do not follow the link to the page being linked tocitation needed, others still "follow" the link to find new web pages for indexing. In the latter case rel="nofollow" actually tells a search engine "Don't score this link" rather than "Don't follow this link." This differs from the meaning of nofollow as used within a robots meta tag, which does tell a search engine: "Do not follow any of the hyperlinks in the body of this document.".

And, just BTW, before it comes up, only well-behaved robots honor robots.txt.