If you have a texture of the wood and you Live trace it that can give you the wood grains depending on the source image of course. Or you can draw some wood grain on paper and scan it then live trace it.

After you have the texture you can outline the text > then using your pathfinder you can turn the text into a compound path. You do this by taking all the layers that have letters in them out of the groups that are formed when you outline then select all the letters and click on (Add to shape area ) this will turn all those layers into a compound path like mentioned before.

Once you have this path you make a clipping mask ontop of the wood texture. But that is probably not what you wanted to do right? or maybe yes? I hope that helped

Or, if your not looking for photo-quality wood, you could just create your own woodgrain in Illustrator and then make it a pattern. Then just convert your text to outlines and fill with the pattern.