I have an iPhone... and a HP netbook. Both are great, but there is little overlap in many situations.
What the iPhone just can't do:
- Take notes (I simply can't input text fast enough)
- Work with Word, PowerPoint, Excel, (look at, yes, edit... no)
- Be bigger. I'm in my 30s, and while I love the iPhone, I can't read a long document on it without having issues.
Again, the iPhone is amazing... but it's not going to replace a keyboard/large screen.