i believe the way it works is you download the isync conduit, which sets up your computer so that your standard address book and calendar syncs go to your zimbra service instead of apple, you set up your mail account in Mail as exchange, and you set up your iphone as Exchange activesync. You provide your own domain, or you use @zimbra.01.com.
I haven't tried it yet, this is just my understanding of the process. they do offer the first 30 days for $1 so you can give it a try.
Thanks for the info. It won't hurt to try so when I get some time, I'll see how it goes. I spent a lot of time last weekend pricking around with MobileMe and it still is so damn inconsistent. Zimbra is a lot cheaper - obviously, you don't have iDisk but hell, iDisk is so slow, and has been for years, it's not worth using big files for.