Some models only or all models?
As said, it's seen like an USB drive by Ubuntu, but the system is designed to sync over the net. Most of the stuff don't need a computer, for example my contacts and phone numbers sync with my gmail and facebook account. Pics with picassa, Files with Dropbox, task with Remember the Milk, etc...
I really only attach it to a computer to make backups when I do system updates (I own a HTC Magic with the latest Cyanogenmod RC). All the other syncing is done magically over the net.
I only tested with one model (Motorola Milestone, called "Droid" in the US). But as far as I know, this is true for every android phone.
It actually syncs very well. It is seen as a USB drive by the system, but Ubuntu finds it has pictures and music on it, and then you can access it through Rythmbox and F-Spot, and synchronize your music and photos with it very easily.
Android phones aren't like iPhones. They don't require a client running on your computer to sync, instead they tend to sync with online services over the air (via wifi or cell network).
Having said that, there is a Dropbox client for Android that will sync your files without requiring you to plug your phone in.
Syncing over the net with google services is much more useful that by cabel. You can sync you calendar, contacts, tasks and much more with Ubuntu, Thunderbird and other apps.