As far as I know file uploads are HTML forms with special input fields:
Code:
<form action="upload.gci" enctype="multipart/form-data">
Choose a file:<br/>
<input name="uploadfile" type="file"/>
<input name="submit" type="button"/>
</form>
when you submit the form, the content of the form's frame will be replaced by the reply of the form's action. So I guess it might be possible to 'lock' the frame 'that is currently uploading' with some sort of a progress indicator.
Maybe there are some issues with HTML, frames and asynchronous requests (AJAX) that make implementing this feature tricky or impossible. There might be lots of problems I'm not aware of. Maybe this is the reason that none of the browsers I know implement something like this?
I'm sure that I'm not the first person having this idea...