Will other manufactures have a AM3+ BIOS update?
I don't see how Asus boards are so different than any other,unless AMD gave only them permission to.
Gigabyte probably already had their boards in production when AMD gave them their Bulldozer Prototype so they chose to finish the production run and deal with it completely with their AM3+ run.
The lines that Asus has highlighted as capable of AM3+ features is pretty small actually. Only 8 boards look like they are able to handle an AM3+ CPU. The M4A89GTD Pro being the cutoff point. And it looks like they've started their production run on the M5A series which will be AM3+.
I'm basing the information for Asus on their Advertisment that was put out by Asus(Taiwan) which is pretty much their HQ. Which you can click on the ImageLink I've provided here:
Hope it clears some things up.And please keep in mind that Asus cannot break their NDA by stating "Bulldozer". If they do AMD has them by the short and curlies.