I've been trying with great difficulty to resolve this issue.
First, I moved a few sites from the web to my localhost (Mac OS 10.6). I also installed a fresh wp site for testing plugins, etc. All sites have the same problem: the media uploader goes through the motions, shows as a successful upload, generates a url, but the image isn't there. It has no thumbnail, either. It does not show errors.
The sites on the web never had this issue, nor have I ran into anything like this, and there are no forum threads showing this specific problem with a solution.
I went through the applicable troubleshooting steps provided here:
No luck. Somebody please make my day!