On packages.ubuntu.com one currently can search for (taking precise as example):
precise (12.04LTS)updatesbackportsBut how can one search for precise-proposed packages on-line? (I know that one can enable the repo locally, update synaptic and look at what changed, but I'm looking for a more trivial solution.)
If you don't mind looking at an "unofficial" site, try ubuntuupdates.org. As seen in the image, proposed and several other filters can be used:
From the about page:
What is ubuntuupdates.org?
Ubuntuupdates.org attempts to give a different view over software released daily for the Ubuntu OS. The information presented on this website is entirely based on data that is freely available at archive.ubuntu.com, launchpad.net, their mirror sites and other software repositories dedicated to Ubuntu. The collection and organization of the package data is entirely automated and no modification is ever made to the text of descriptions or changelogs.
Ubuntuupdates.org first came to life in October 2009. For those curious about it's internals, it's entirely implemented using the Ruby on Rails framework.