Is tree view available in Finder in Lion?
The way items are shown in Finder can be configured through the folder's Toolbar, which gives four default options:

 


Icon View - Which is the regular layout of icons. 
List View - The items are shown in a list-like and can be navigated as a tree.





Columns View - Items are displayed through columns which show the file's path.





CoverFlow - Items are shown with the regular coverflow efect.


Note: While in a folder, you can switch through these options by doing Command + the option's number (1 is the icon's view, 2 is the list view, etc).