Did you try pressing the red phone key or the "OK" key? Typically, for our WM6.0 HTC devices, it is possible to change to the today screen while uploading. The CPU load is between 50% and 100% during that process, but you still can use other programs.
On my setup (HTC Touch Cruise running WM6.0) when I upload a training to the portal (via activesync or wifi) my PDA is locked up during the upload. If you have many pics/vids this can mean 10-15 min when you can not do anything else. I would like this to be done in the background. I realise that it uses lots of resources, but it should not lock the machine up completely