I got tired of not having fire weakness/resistance apply to lava, so I whipped up this little mod that fixes that.
http://oblivion.nexusmods.com/mods/43046/

It's pretty straightforward stuff.
It's all scripts, so it should be compatible with everything, including mods that alter Lava textures.

Give it a whirl.

Ask questions if you have any.

saebel