Frank Kromann applied some patches yesterday to the PECL sources of the uploadprogress extension to make it compile under Windows (almost the same as by SteveH provided in a comment of my post yesterday). Therefore, thanks to the pecl4win site, a compiled dll is now already available. Get it here (untested by me..)
And about the documentation asked in the comments, there are some hints in my earlier post. In general, it works exactly the same as the old patch, which was available for 4.x and 5.0/1, except that the only function available is now called uploadprogress_get_info() instead of upload_progress_meter_get_info().
I may write some documentation, which shows plain and simple, how it's supposed to work, but can't promise anything any time soon. But as usual, if anyone else steps up, I'm more than glad to link/include/mention that.
Update: Due to popular demand, I put together a simple example (I tried to make it really simple), all the needed files can be get from here.
Update: For unknown to me reasons, it doesn't really work on windows. I have no idea why and don't have a windows developement environment, therefore I won't/can't fix it. But everyone is invited to have a look at it (see also the comments).
Update 2 years later : There's now an āofficialā release working on windows. See this blog post for more info and this blog post for info about general issues with the extension.