It's not only iphone any smart phone will do.
There are plenty phones out at the moment, blackberry, HTC, Nokia just to name a few, the iphone has the most choice of Apps.
I am setting up one soon. I am not sure who to choose as yet I am going for the cheapest.
One of the most popular is Intuit GoPayment from the company that makes Quickbooks, google it and check out the company.
Also Check out:
MobilePOS - from Adelante
http://www.adelante.co.uk/mobilepos.html (need a merchant account - I am waiting on them to tell me if it can link with PayPal as the merchant.)
Always check the Setup Fees, Annual Charges and cost per transaction, oh and the minimum trade value per month.