I've just completed my first implementation of 3.0.1 in multisite mode. I have previously made about 12 installations of various versions over the last three years without any problems, so I am no expert, but do have some experience.
The problem I experience in this installation (same server as three other 'normal' installs which work fine) is that when you click on the insert image button on the post edit, it opens a new page and though the upload works, it just adds the image to the gallery and I have no way to insert it into the post as intended.
Here are two screenshots - the edit post and the new page which opens when you click on insert image:
Many thanks for your help solving this!