Here's a list of specifications in Ubuntu that are well written or which illustrate a good approach:
Joel Bryan's Ubuntu Live Chat Support spec. This one uses a LOT of screenshots to explain exactly what is intended and how it will feel for users. It also includes detailed information about the structure of the IRC community, as part of the plan to point people at the right channel for their language.
Matthew Thomas' specification for the Dapper Graphical Installer UI. This specification does a great job of drawing up what the different screens in the installer need to do. Note that this spec is part of a tree of dependent specifications, so it just focuses on the user interface issues, leaving the rest to other specifications. Also notice the commentary added by other users.
Daniel Holbach's "Bug Best Practices" spec. Here, Daniel was trying to organise a rearrangement of existing information in the Ubuntu wiki into a better layout. Note that the spec says it's done at the end, which stops further commentary.
Carlos Perello Marin's Rosetta Import Process spec. This one uses the combination of a lot of database schema specifics, as well as screenshots and mockups, to give us a clear sense of what is being planned.