You need something that uses WebDAV if you want to sync with the iPhone. Probably the easiest, cheapest route is to use the Omni Sync Server (
https://sync.omnigroup.com), though if you've already got MobileMe you can use that. The Omni offering is free, works well for most, and the people running it understand the format of the database and can help fix things if something goes wrong. Billy Bob says check it out :-)