Recently, we changed the way assigning bugs works on Launchpad. It used to be that anyone could assign anyone else to a bug. This was open to abuse as you can imagine. Bug 511269 was filed about the potential problems with this, and we recently changed Launchpad so that only bug supervisors can assign a bug to someone else.
You can still assign a bug to yourself, but this does keep you from assigning someone to a bug to draw their attention to said bug. In the end, this is a good thing, though, as people should only be assigned bugs who are going to be responsible for working on them.
Now there is one issue with this change. Projects that had not established a bug supervisor for the project will find their developers can no longer assign bugs to each other. The easy fix for this is to create a bug supervisor team for your project and have the people working on your bugs assigned to this team. We do realize this might be a bit heavy weight for some projects, so we’ve opened bug 603281 about this issue. A fix for this — only requiring bug supervisor permissions if bug supervisor is defined — should be appearing on Launchpad soon.