First and most obviously, Yahoo! is the only Web-based email/contacts store supported: If you use Hotmail, Gmail, AOL, or any other Web-based email service, you cannot sync between contacts stored there and the iPhone. This is a glaring functional lapse that the early Mac-using iPhone reviewers neatly skipped over
Boy, iPhone users sure are screwed, especially since, outside the Helio Ocean (whose site isn't clear on whether it actually syncs with Yahoo!), I can't think of another mobile phone OS - including Windows Mobile and BlackBerry - that syncs with a web-based email or contact service out of the box (note: sync - not simply 'allow access with some custom UI'). At least, not a significant or even publicized phone from any of the big players like Nokia, Motorola or Samsung. The simple fact is that most mobile phone OS developers haven't made that leap yet, primarily because the web-based services like Gmail and AOL haven't opened themselves up through an API (Application Programming Interface) like Yahoo! has for the iPhone. This is probably because, in the past, it hasn't been worth the effort. Most users who want to sync their contacts with a mobile phone are either power or business users, and they're already using desktop software like Outlook, Entourage or Apple's Address Book that is primed and ready for synching. Apple likely took a chance and opened this partnership to sync with Yahoo! because the iPhone is arguably the first consumer-friendly phone to bring the concept of synching to the general user. For those still wondering why Apple chose Yahoo!, it's likely because they are the leading worldwide webmail provider by a landslide; as of April 2007, Yahoo! Mail's market share doubles Hotmail's and, believe it or not, Gmail trails in an incredibly distant 3rd with 1/13th the traffic of Yahoo!.