This is asked every week it seems and via BES this is not possible as of yet. It was discussed at WES and is planned for BES 5.0.
That being said there are a couple ways to do this with Web Desktop Manager being your best bet if you have a number of devices. There is also the nifty Blackberry upgrade webpage. We unfortunately lock down users to install anything so it's not an option but would be easy for a small company / less security as you could direct users to this website when OS updates are available (and save your company bandwidth)