Tabs in Xcode 4 are awesome. Each one maintains its own separate layout, so you can have multiple tabs with different things in them and have each one lain out appropriately.
I've been setting up my projects with three layouts—one for the project object, one for each nib (showing the owner's class's sources), and one for each class.
There is one bug here: As of 4.0, Xcode will reset the state (turning off any Assistants) of the active tab when you reopen the project. The workaround is to switch to a tab with no Assistants for Xcode to close before you close the project. I have filed this as Radar number 9178441.
