Drupal is by default last time I checked, but pretty much anything can be with plug-ins or minor code changes.
Make sure you comments are relevant b/c no matter if it is do-follow or not most are default moderate comments so the blogger can easilly delete spammy comments.
You can make any blogging platform dofollow. Wordpress is easy to do with a plugin. Blogspot requires template modifications.
Yes thats right, just you need to tweak the coding in your post.php(wordpress) or whereever you want add add it rel=nofollow in hyperlink tag.