Emerging ipython 0.12 with USE-flag notebook needs to pull in tornado in at least version 2.1.0.
Created attachment 298385 [details, diff] Patch to correct DEPENDS variable in ebuild.
working on a new revision for a bunch of bundled dependencies.
You can use dev-python/ipython-0.12 from Progress Overlay, which also doesn't contain many other bugs.
fixed in ipython-0.12-r1