I have experienced this problem on several configurations and have web development clients who also experience this problem. The "Send to Editor" button works fine for awhile and then suddenly stops. There does not seem to be any consistent set up for the problem. I've had reports of the problem on everything from Mac OS 10.4.11 to the latest OS X with Firefox (the last 3 revisions) and Safari (last 2 revisions), from Windows XP to Vista with IE 5 on up.
It seems to happen within 3 months of initially setting up WordPress on a site.
I have found a few of things that appear to work temporarily to clear up the issue:
1. I start by clearing the browser cache.
2. Shut down the browser and relaunch. (Often this does the trick)
3. In the user profile, unselect "Use visual editor" option.
4. Make the change from "win = top" to "win = parent" in upload.js file.
Reinstalling or upgrading files does *not* make any difference normally.
I know it is very, very difficult to debug an inconsistent problem, but this has been going on for at least two full upgrades to WordPress. It's forcing me to look at Joomla for clients because they Freak Out whenever they have a problem with the editor.
(And yes, I do make clients donate to WordPress.org whenever they choose to use it. It's the right thing to do.)