I think upload resume would be pretty impossible, since the recipient has no idea of exactly what is to be uploaded, and would have no way of knowing what has/hasn't been done without fully scanning the file - if the file is already on the server (as with downloads) then a cookie etc could be used, but with the file being on YOUR computer instead, the server would need to scan the file completely, which would require it to have the file - in other words it would have to be uploaded already
Maybe someone has come up with a way around it, but i've never seen it.
As for the ability to resume - it's much harder than it looks. I'm not 100% sure it's possible using JS without doing things like requiring cookies, which is something that should be avoided where possible really. David will look into it when he is coding the next version of vBDownloads though =)