Image links not working on fresh WPMU install

I have been struggling with an issue that has cropped up in the past with WPMU for me.

I've set up my site pretty much as instructed. There was a .htaccess file created, which is identical from the dist version except that it has the word "BASE" removed from it - that's it.

Now, once I log in to my primary blog and upload an image, the link is broken. It tells me under the image details that the image is at the "shortened" path - in /files/2009/12/10.jpg - but not that it's actually in /wp-content/blogs.dir/1/files/2009/12/10.jpg. I can see that the file is being uploaded into the correct place, but the links to the image always break. If I enter the long (correct) URL, I see the image fine. But I suspect I'm not meant to be seeing that long URL.

I suspect that the problem is with my .htaccess file but I can't understand what could be causing this, nor why it would be happening to my stock install. Are there other files that could be implicated here?

This is a fresh web server running Apache 2.2, which I also just set up with basically a default configuration. I should add that subdomains (as separate blogs) work and have exactly the same problem.

I have added the wildcard * A record to my DNS entry for my server, and I have set up the ServerAlias in my Apache correctly, and restarted it.