I swap devices all the time. I did use a Blackberry Pearl for a while, which I have dropped a couple months ago. I got my first iPhone last September and I am actually looking forward for the next generation, as given I live in Europe I am very much looking forward to the 3G model. It's probably the best new device I have ever had, especially in terms of reliability and stability, which is amazing for a newbie company in the market. But then, it's Apple. (typing this on my MB Air, perfect machine for my purposes!)
The other device I currently use is a Nokia E61i, as for mobile email extended typing is somewhat easier with a real keyboard, especially while walking or similar.
I have used IMAP for eMail on the iPhone and I am syncing it for calendar and contacts using iSync/iTunes on the Mac, which includes, through CalDAV in iCal, my Scalix server-side calendar items.
We are currently not planning to do any specific client development on the iPhone. Our Mobile Web client, included with Scalix 11, does allow one to read and respond to Scalix eMail using the Safari browser.
We will be publishing ActiveSync support later this quarter, hopefully in time for Apple's launch of the 2.0 software. We will include this in our ActiveSync testing and you can rest assured that I won't let this software out of the building before it works perfectly with it, because that's what I personally plan to use going forward. If, from a user experience perspective, this leaves anything to be desired, we will certainly consider providing additional software to run on the device, but in general it is our strategy to utilize open (like CalDAV) or accepted industry (like ActiveSync) standards and do as little development on the client side as possible.
BTW, a couple of our commercial customers have also asked about the above, so the interest for the device is quite strong in our market segment. I am quite fine with it.
HTH,
Cheers,
Florian.