If you're making $150K/yr, why worry about a tiny stimulus?
The idea is to get money into the hands of those who will throw it away right away. If you give $600 or $1200 to a family that's making $150K, it won't affect their spending, because it's not enough money for them to change their behavior over. They're not going to say, "oh, goodie, now we can go have a really nice dinner and spend some time at the spa now that we finally have money!"
If you can get this money into the hands of people making $30K ($15/hr) or less, you'll have a much higher chance that they'll get the money and then go out and spend it on something they would not have otherwise bought. These are the exact people the government targets with this money. At that income level, $600 (or $1200) goes from being a couple days worth of income to the families and individuals on the edge of receiving the money to a over a week worth of income. That's enough to affect spending.
Also, there is confusion in this thread on how the package works. It's an advance on an *additional* 2008 tax break. So it's money the government is going to give you back on your 2008 taxes which you'd normally get in early 2009, but because they want it to help the economy now, they're giving it to you in advance.
