If you are firefox user you can install a plug in "SearchStatus" that will highlight the no follow link in that site If there are no highlight that mean that site have a do follow link see their homepage http://www.quirk.biz/searchstatus/
Sick with the list previously mentioned and you will do well. We check the SEO/SEM Sites weekly and the other sites monthly. The tag will not change unless someone changes it.
You can find dofollow blogs matching your keywords using a tool 'Fast Blog Finder'. The link is in my sig.
Some of the dofollow blog finder tools are pretty useless... I do like the dofollow diver from Linkbuildingbible, and I've found some gems with that one. courtneytuttle.com has a really good list of dofollow blogs as well. Just be careful not to use anchor text if other comments don't have it... cuz usually that means they moderate and will not allow it. If others are using anchor text links then you can get away with it, because they probably aren't moderating the blog as closely.
High PR doesn't always help... with regards to dofollow and nofollow, webmasters can make their site dofollow or nofollow at anytime they want,but there's no automatica timer or anything.
Interesting concept. Here are some dofollow blogs that are easy to find (hint they are right below ) www.web-reviewers.com www.freeiqz.com www.bodybydesignonline.com They are all PR3 as well and have very little comments so far.