Uploading of images in the editor window gives me "errors on page", probably when WP tries to give me feedback as to the loading status of the picture.
The picture does upload though. I saw this as I tried to work-around the error by uploading with ftp. I saw the files already there. I figure the error occurs when WP tries to display the "thumbnail - link" options after uploading. (I extrapolate this from the previous version, 2.3.3.)
In short: the image upload function doesn't work properly yet. This might be a good one to fix in the next release.