if you are using wordpress, there is a plugin specially designed for this, called Dofollow. you can download it here: http://wordpress.org/extend/plugins/sem-dofollow/
No. But... If people can be bothered to write a decent comment on my blog, I'm happy to give them a 'do follow' backlink. What goes around comes around... I use the the plugin that darkblue09 posted, it works
I removed dofollow on my blog. I was on some "dofollow blog lists" being sold here on eBay and I just got spammed like crazy... no follow it is!