Here's my guess as to why Rove is still under investigation. The indictment comes out against Libby. He will now be offered a cooperation agreement / plea deal to reduce his sentence to finger Rove, and maybe others.
There are a whole lot of soggy bottoms in foggy bottom right now...
And as a side note to my theory. If libby were a clinton administration official, he'd be in fort marcy park right now so he would never even have opportunity to sing like a birdie.