As a med student (and soon to be physician) that owns an iPhone, the only reason I still carry my Palm T3 around is for medical applications. That being said, here's my list of most wanted applications for the iPhone:
-Epocrates
-Dictionary/Thesaurus
-Pocket Wikipedia or other Encyclopedia
-Full featured alarm clock
-Google chat
-Medical calculator
-Netter's anatomy atlas (full resolution)
-Tarascon pharmacopia
-PDF/image viewer such as Preview
-Ebook reader
-Word/Excel document reader
-Keynote document viewer (also Powerpoint)
-Full bluetooth functionality (sync, beam contacts, caller id displayed on my Mac)
-Islamic prayer time calculator with calendar
-Restaurant guides
-World Atlas (political/geographic maps)
-Copy/Paste functionality throughout the iPhone
-Advanced notebook/scrapbook
-Astronomy planetarium/atlas
-Voice recorder
-Ability to save/archive voicemails as mp3 files
-Spore
-Lunatic Fringe (I can dream can't I?)
-Crosswords
-iPhysics
-Unit Converter
-Scientific Graphing Calculator
I'll add more as they come to me...
User Poll: What Do You Want from the iPhone SDK?