Here is the latest snapshot of the current TeamCity state: build #13245.
One of the major improvements introduced in the build is new UI for editing build configuration triggers. We migrated from tab-based triggers UI to list with add/edit/remove operations as this is necessary for further extending triggers capabilities and configuration.
See the other changes in the release notes or browse through the list of fixed issues in our tracker.
Please try the build and let us know if anything goes wrong - 5.1 is within weeks, so we need your help to discover any hidden issues.
And just in case, do not forget to backup your existing data before upgrade!