Vista is not the preferred Windows OS for a netbook. I don't think it is even preferred for a desktop, certainly not for corporations. Unlike the move from 98 to XP- with features like System restore or Driver rollback- we cannot really point to any great advantage to migrating toward Vista.
For one thing, it is too expensive and as you say, it needs a lot of RAM. MS dropped the fee for XP to netbook vendors as they noticed they were missing out on a new, important market. But, to keep prices low on these items, they were forced to drop the price of XP to enter the game.
Yes Win 7 has even been optimized for netbooks. In one hands on of the beta I read about, they were able to open 100 windows without getting a low resource warning. So, Win 7 can handle memory usage better than Vista, as Vista uses memory in a linear fashion, increasing depending on how many open windows you have.
Battery life or optimization is reportedly a focus as well with 7. MS recognised that Linux was a bit of a threat on these small notebooks. They also see that notebooks are outselling PC's right now. So, I guess they are focusing on that segment.