I didn't get much response to this directly but I notice that a number of problems have been identified and fixes are ongoing.
I'm trying now to identify precisely which fixes we need to make in order to get the first real RC out for 1.5.1. If you've been trying to build 1.5.1 and are having problems can you please raise JIRA against the 1.5.1 category so we can track them. Also if you are doing general fixes please make sure that they go into the 1.x branch and not 1.5.1. We need to be trying to stabalize 1.5.1 without introducing more changes unless necessary. Thanks Simon