The phones OS/platform is more important today than it ever was before, and will be even more important in the future. When I look at buying a new phone, one of the first questions I ask is what OS is it running. However, I disagree that the phone OEM will become irrelevant or disappear. People trust certain mfgs more than others. I wouldn't hesitate to buy something from Nokia, SonyEricsson or Motorola. If the phone was made by LG or CECT or some unknown mfg, I wouldn't buy it. If the phone was only branded by Google, I would ask who the mfg was and then decide. I think the truth is in the middle. I think the platform/OS makers will have to work closer together with phone mfgs for both to be successful. Like the Motorola Droid as an example. People know/trust Motorola, and there's trust in Google/Android, so it came out to a good match.
I can't have OEM go completely. My mother knows how to "drive" a Motorola. I think if the OEM Motorola OS significantly changes she may have difficulty using a new phone.
That Moto OS is the reason I never bought a Moto after my original RAZR. But now, IMO, Moto is back in the game with Android. I really like my Cliq, and I also think Moto did pretty well with the MOTOBLUR overlay.
I bought mom a pink V3xx last spring, and no significant retraining of my mother was required. I'm happy.