Much has been said about why 99% of AM2 motherboards don't support Phenom. One of the main reasons pointed at is that boards need an 8Mbit BIOS chip, to include additional code that doesn't fit in a 4Mbit BIOS.
That is incorrect.
I managed to dig up three boards from Biostar, with a 4Mbit BIOS, that do have Phenom support, with reports that it is working well. From the user Hemi:
Pros: I have 2 of these boards and they are great boards. One is running a 6400BE, 4 gig of ram at 3.360mhz air cooled. The other board has Phenom 9500 running at 2.5mhz on my bench. air cooled....
Other Thoughts: Bios file A09 works great with dual core.B06 works with quad core and 6400BE and may or may not work with other dual cores.A09 is what I use for 6400BE. These are listed on Biostar web page for the TF560 A2+ ...
He further confirms that, like Biostar points out, you need to update the DMI table. Not update, though, but simply clear it. Kindly though he also explained the necessary process, usually easy to do with Award's WinFlash:
Cons: biostar does not explain how to update dmi using onboard flash program.Copy bios file to floppy.When you are ready to flash using flash program,you will see in red (highlighted)your bios file on the floppy.Use TAB key one time will highlight DMI.Page up or down to change to yes. Tab one time to return to bios file. Then enter to start flash program.If using Dos to flash you have to use switches to make it work right.I hope this will help you guys running a dual core and are ready for a quad. This should help Newegg and you, not having to rma because of bios problems.Just remember to update DMI when flashing or you will have problems with not booting or getting into windows...
This could be the reason why Patrick Schmid and Achim Roos, from Tom's Hardware, ran into some unexpected problems with that Gigabyte board, although it had an 8Mbit BIOS chip. Like they claim, the pre-production version of the board is the most likely cause, because this DMI issue seems to exist only in the TF560 A2+.
The other two boards from Biostar that have Phenom support with a 4Mbit(512KB) BIOS are the TF570 SLI A2+ and the TF520 A2+.
Remember that these three boards all use HT1.0 and don't feature support for split power planes. All of Asus boards that did get support had 8Mbit BIOS chips in them.
Biostar also lists the TA770 A2+ 5.x but this one already uses an 8Mbit BIOS and is an HT3.0, fully AM2+ board.
The support of Phenom with a 4Mbit BIOS goes inline with Biostar claims and user's feedback. However they aren't listed in AMD's list of recommended motherboards. Usually AMD doesn't really certify that many hardware, so this isn't significant at this point.
DFI also has a beta BIOS for the LANPARTY UT NF590 SLI-M2R/G, fitting in the 4Mb cap.
Jetway lists some boards as supporting 45nm CPUs, but it is a huge typo. Phenom support is not listed and the closest to 45nm out there is 45W CPUs, which is already there in the form of "Support for BE & LE" CPUs.
The PA77GTA-VT does refer the Phenom, but it is an HT3.0 board with an 8Mbit BIOS, like the Biostar mentioned above.
MSI has also released a list of supported boards but, from what what I could uncover, they have 8Mb BIOS chips. An example is the K9N Neo V2 BIOS.
MSI's site requires an actual MSI motherboard for Live Update to properly work, which was unavailable for testing. This BIOS isn't the one with Phenom support, but it serves it's purpose.
I initially thought the more recent MCP65 chipset used in the Biostar boards could play a role here, but the support in the DFI board clears that up, since it uses an MCP55 and a C51.
The manufacturers do have the necessary hardware to have a broader Phenom support, they just don't seem worried about it and it could be for an array of factors. Fitting a BIOS with Phenom support in 4Mbit may be close to impossible, very hard, but whatever can be claimed, impossible it is not.
Biostar promised to deliver support and they eventually did, even with the 4Mb restraint.
The only thing that comes to mind able to help offer support for Phenom CPUs - if manufacturers don't provide it - is the Coreboot project. Previously known as LinuxBIOS and already working fine in some motherboards, including the M57SLI-DS4 from Gigabyte, it aims to provide a free, open-source BIOS. That could mean Phenom support in more boards with 4Mbit BIOS chips, but it will take some time until it is production usable.
Coreboot already supports the Phenom and fitting it in 4Mbit is a possibility also. I'm waiting for further confirmation about that.
More information on this subject is scarce since both AMD and motherboard manufacturers haven't really given out much.
Money plays a role here, since supporting an array so big of boards would be expensive, but manufacturers are the ones who have to speak up about that.
Update: Marc Jones, Senior Firmware Engineer at AMD, has confirmed that coreboot should provide functional systems with Phenom support, with the regular 4Mbit coreboot BIOS. Work is underway, since not many motherboards are supported at this time.
Update 2: ASRock has done it, in a 4Mbit BIOS, for an NForce 3, even.