That's still an open question. To the best of my knowledge Microsoft
removed the ability to embed Javascript by using a Content Editor web  part
on purpose. But I haven't really seen an official comment on that omission.
But its definitely gone and I haven't seen any alternative at this point.

It was mentioned at the SPC09. It was disabled due to the risk of cross site
scripting. It can be reenabled though. I recall it being in the properties
of the part.

You can embed javascript using a content  Editor web  part.
There are certain limitations like embedding a startup script or declaring
variables in some cases. There are workarounds as well like you have to push
your startup functions to a function stack and it gets executed.

It is not straight forward to get the source editor  popup.As you know content
editor webpart now allows inplace editing.Put your cursor in editor and wait for
couple of seconds so that Editing Ribbon appears.
Under Editing Tools -> Format Text you will find "HTML" button under "Markup"
group. Select the dropdown menu and select Edit HTML source.

How to edit source in Content Editor Web Part in SharePoint 2010?

Complete step by step instruction can be found at virtualizesharepoint.com/.../