It's interesting when we measure new things using old rules. A number of people have observed that Vista machines doing nothing seem to be using a lot of memory to achieve that nothingness. Words like "bloat" get bandied around. Empty memory is seen as more virtuous than filled memory. I'm not going to link to all the "Vista is using all my memory it sucks" complaints. Instead, I'm going to point you to Jeff Atwood, who explains the whole thing quite nicely and concludes:
The question shouldn't be "Why does Vista use all my memory?", but "Why the heck did previous versions of Windows use my memory so ineffectively?"