That question depends mainly on the Total Cost of Ownership and how fashionable you need to be.

Phones such as the iPhone 4 or Samsing Galaxy S etc aren't inexpensive. Typically an iPhone 4 unlocked in the UK is getting on for around $800. Take into account call, Internet usage including App downloads and insurance then the cost is pretty prohibitive for a phone that isn't even ruggedised.

I've basically settled on an unlocked Samsung B2710 for 1/6 the cost of an iPhone.

http://www.youtube.com/watch?v=-HXNZmIJ5VA

The Samsung should still work if going for a post Katrina swim for example.