BlenderGuru.com's Andrew Price has released a tutorial on realistic fire as an ebook. To get it your hands on it, you have to sign up for his maillist (which informs you of his new tutorials). I realise that this is a barrier for some, but I found the ebook to be very nicely made, and I trust that Andrew won't do funny stuff with my address.
I've finally finished my tutorial on Creating Realistic Fire in Blender 2.5:
I’ve been working extensively with the smoke simulator over the past few months to find out the best method for creating fire. It’s been a long road but I believe I’ve finally got the formula right.
I'm releasing the tutorial as an eBook... and it’s completely free.