I've had an iPhone for the last couple of years and this summer upgraded to the 3GS. Yes it is quicker and you can do more with it and these are reasons I why I think every illustrator/designer needs an iPhone:

Email: The number one reason I got it and the feature I use most. Since I want to keep my flexible career, being reachable is key.

PDFs: This was the reason I switch from using a palm phone to and iPhone. iPhones can read PDF files and since a lot of my jobs specs come in as PDFs I really needed this feature. When I had my Palm phone, I would have to wait until I got back in the studio to respond to an email about a job, or while on vacation, I would have to call someone to log into my webmail and tell me what the PDF said. When you work in children's publishing, deadlines are tight so people usually want a answer that day.

Social Media: I stay connected through twitter and facebook through my phone while I'm out of the studio.

Mobile Portfolio: I have a collection of work pieces within my "photos" area that can be easily scrolled though if I need to show someone my work.

Apps: There is seriously a app for anything. The app store is chocked full of great apps and many of them are free.

The only downside I've found is that they are not able to view websites in flash, but I can live without that.

And personal reasons you should have an iPhone - especially if you have kids:

Video & Audio: You can record on the fly and have a record of cute voices forever:

Movies: It also comes in handy for watching movies during those long restaurant & doctor waits (I currently have Nemo, Cinderella, Dora, and various tv shows on it for just those occasions)!