Title is basically it. I have tomcat running on localhost:8080 and I'm attempting to use IIS 7 with ARR to reverse proxy to it, which half works. The request is getting to tomcat but with the Host header of 127.0.0.1:8080, which the servlet is complaining about.
ARR does a reverse proxy request to the target Server Farm, which is localhost:8080 in your case. When it does, it writes the original server variables into HTTP_X_?? variables.
From tomcat, you will need to rewrite those variables back into the original locations if you want to ensure that applications on tomcat are affected by the reverse proxy affect. If your site was IIS7, you could use the ARR Helper which does that for you, but since you're not using IIS, you'll need to handle that manually yourself.