Kareng-
Here is an interesting article on dyslexia and learning a foreign language: http://www.ldonline.org/article/6065/
It is an old article (1997) but I think it makes some good points.
I would suggest, as long as it doesn't cause your child undue stress, to go ahead and attempt a foreign language. The extent to which dyslexia hinders the adoption of a foreign language seems to vary greatly depending on the individual. For example, I know a dyslexic college student who is majoring in a foreign language. Of course, the amount of struggle involved depends on the severity of the learning disability, but it is possible and there are teaching techniques that work better for the dyslexic mind.
Best of luck,
-Learning Ally Mom