I take pictures. I use Lightroom to process them. I like it. The current camera emits DNGs that average between 20-25M, and I take them dozens or hundreds at a time. Lightroom 4 is, um, not faster than Lightroom 3 and I see no evidence that Adobe knows how to make any future version faster. Lightroom on the current machine is just barely fast enough, where "just barely" means "not really".
I have the same problem: my old MacBook is slowly dying and can no longer run Lightroom 3 (or Xcode for that matter) in an usable fashion. I download the trial version of Lightroom 4 and it is worse.
I am looking to upgrade to a new Apple laptop. My ideal machine would be a 13-Inch MacBook with Retina Display, SSD (+ external HDD), at least 8Go and the latest Intel processors. I am waiting for the next generation of MacBook Air and Pro to be released to check if one of their models matches my expectations. However, I will monitor comments on Tim's post in case the next Apple laptops do not change that much.
In the mean time, I am stacking up photos until I can post process them...