After upgrading to 10.1, I now get a prompt "Choose a Mathlink program to launch" when I call code that uses ExcelLink. This code worked prior to the update. I am wondering if this is a configuration setting. Is ExcelLink looking for Mathematica 10.0.2 (which is uninstalled during the 10.1 install).
Turning @ilian's comment into an answer:
This is a bug, which has been fixed in the development version. For 10.1.0, you may contact email@example.com for a modified ExcelLink.m file, or use the patch from http://community.wolfram.com/groups/-/m/t/472231