Hate to say this, but I pretty much expected as much.
Personally, I suspect the .Mac infrastructure was FAR less than sufficient, for quite a long time (probably ever since its introduction). Apple was coasting along with it because a relative few people made much use of it.
I know back around 2001-2002, I was doing a lot of web development work for a client who hosted everything from .Mac, and I was appalled at the poor performance. Whether it was from my point-of-view as the content creator/editor trying to slowly upload things to the iDisk, or as an end-user, seeing .Mac hosted sites SLOWLY pulling up, even on fast broadband connections -- it just didn't look good for Apple.
I also know that on at least a few occasions, this same client received notices from Apple about email outages and issues too.
When they announced this MobileME launch, I thought, "Wow... Apple's getting pretty ambitious with this whole thing. It's going to take a LOT of infrastructure improvement if they expect this to work right." I think they tried to re-use too much existing hardware and bandwidth, and what was just "poor" before suddenly became totally "unworkable".
I agree that they might actually get it all sorted out this time around. They have too much at stake now. (Many iPhone purchasers are going to rely on this as a primary reason they can justify using the phone ... so Apple can't afford to just leave it "limping along just well enough to make a little profit off the service, and say they have it".)
Growing pains. But they'll get the kinks worked out, and it'll be awesome.