It appears I am not the only one having problems with WP-United after updating to WordPress 3.1. There is a forum discussion on this issues and the same solution is proposed as I would have attempted. The proposed solution was to change the function definition in WP-United to a different name, as the error indicates a function naming collision. According to the forum, this allows template integration to work but user integration fails. I’m not sure that the comments integration would work in this case, so I don’t think this is a viable option. Time to look at other possibilities.
Here is another blog with the same problem and solution.