I too would be very interested to learn that auto-bracketing was in the cards as an update for the D3100. I use my D300, but my daughter is ready to be an entry-level SLR kid and she is always so enthralled by HDR effects that I can not see purchasing one for her without that feature. The D5100 is out of the price range.
Nikon virtually never indicates its intentions before announcement, so anything you read about this is at best speculation. In fact, Nikon's practice is that it barely ever issues firmware upgrades. Moreover, I can think of only one instance where a firmware upgrade added significant new features - that was the firmware release that supported the D2xs, which added most D2xs features to the D2x. Other than that single instance, virtually all of their firmware updates were to fix bugs.
So I'd say that there is little to no prospect of a firmware upgrade adding auto-bracketing to the D3100.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!
Welcome to Nikonians! It depends on the perameter that you want to vari when bracketing. If changing the shutter speed works for the situation the results should be the same. Good Luck and Enjoy your nikons!