I'm not sure I'm convinced of this, in the case of Asus (or Gigabyte or Intel, who have sponsored some LinuxBIOS/Coreboot projects in the past). Asus, with their Eee PC, are striking out into delightfully new territory, and an instant on feature would be a pretty killer thing. Traditional BIOS makes that impossible, because it's so damned slow and changing it is, as you note, practically impossible.
But just because the BIOS software companies (who are, on the whole, separate from the motherboard manufactures) don't want to rewrite their software, it doesn't mean some motherboard manufacturer won't jump ship one day. In fact, I would argue that it's inevitable, if the BIOS manufacturers don't get their act together fast--people are beginning to expect a completely different experience from their devices. The iPhone and similar devices are training people that real computers can turn on instantly and be ready for work in seconds. No reason PCs can't be similarly fast to boot.
But just because the BIOS software companies (who are, on the whole, separate from the motherboard manufactures) don't want to rewrite their software, it doesn't mean some motherboard manufacturer won't jump ship one day. In fact, I would argue that it's inevitable, if the BIOS manufacturers don't get their act together fast--people are beginning to expect a completely different experience from their devices. The iPhone and similar devices are training people that real computers can turn on instantly and be ready for work in seconds. No reason PCs can't be similarly fast to boot.