People have been whining for this for quite a while, and although there was a patch available, and Josh showed how to use it with html_ajax most people don't feel comfortable patching and recompiling php. And if you're like me and think compiling on windows is only for the insane, well, you're outta luck.
After a lot of complaining from people on the php.internals mailing list, the hooks for doing upload progress are in 5.2, but they need an extension to work. This is the first one that fits the bill (I think they need at least one upload progress meter extension in the core - even just a simple one)
Now, it hasn't been tested on windows - but after it's in pecl if it will compile (thank you edin and pecl4win) I'll be jumping to try it out (I'm weird like that). This is something I've been looking VERY forward too, and if it does cooperate with windows I'd like to hack together some file upload support for html_ajax.