Coming from an iPhone, I already have an established library of my music. I found 'iTunes Agent' that integrates with any device (Not just the Nexus One).
It lets you sync your iTunes library with a folder basically.
It isn't that simple though, you actually have to sync a playlist from iTunes that the iTunes agent actually reads and uses. It does setup the playlist in iTunes for you so you can just select all your current music and add it to the playlist and it will sync.
I'm finding it to be an almost perfect solution to sync my music over. The only downside is that my tags aren't stored in the files themselves it would seem and only in iTunes. This causes me to have duplicate albums for songs tagged with different album names (slightly different titles etc).
Hope this helps.