Okay, I've heard here a lot, that people want simple integration/syncronization with iTunes, iPhoto, iCal, & Address Book. These are all, (minus iTunes) 100% Mac-Centric. PC users would only get integration/syncronization with iTunes. What good is that to them? At that point you only have iPod + Phone.
So Apple has a choice: Mac-Centric or not.
Knowing Apple, their first choice is "not" (which doesn't mean it will start out that way, but we'll just have to wait to find out). Apple would then have to either write iCal et al. for Windows or build in support for Outlook, ...uh... photo viewer... whatever PCs use for photos.
Both are daunting tasks.
Conclusion: In order for Apple to make a phone as good and as universal as the iPod, it will have to accomplish one of the aforementioned daunting tasks.
Making a phone for Mac users would be a walk in the park, because 1) it's such a small microcosm, 2) It's an environment that they are familiar with.
Making a phone for everyone will not be as easy. HOWEVER, Apple is great at building OSes (the iPod OS is simple & intuitive and I have no doubt that they will do the same with a phone) and Apple is great at integration with software, so even though there will be hurdles to overcome, Apple will eventually churn out a phone that is simple and is loved by everyone.
I also think there won't be a single serious Mac-User who won't have one. It'll just be too handy to have a device that will sync easily with the awesome Mac software.
-Clive