Can you use iPhone without a data plan?

Basically, you cannot. In most cases, when you purchase an iPhone 3G, you sign a contract that includes a data plan for a certain period of time (in the US, that's two years).

Get yourself an iPod Touch.

The iPhone is pretty pointless without the data plan and in the US, the data plan is a requirement.

Without the data plan, you pay per kb for visual voicemail, apps that access data, maps, email, internet, etc.

The iPhone is designed to always have an internet connection as long as there is an available wi-fi network that you have access to, or you have cellular reception, and the iPhone switches back and forth between the two automatically and seamlessly.

If you do not need, nor can you justify paying the monthly expense for the iPhone's data plan, then there is no point considering nor purchasing an iPhone. You should consider an iPod Touch instead.

I'm in the UK & use my iphone without a data plan. I purchased my phone from Apple I then inserted my regular O2 sim rather than the 'Pay as You Go' sim supplied by Apple. I am able to use email & browse the internet at wifi hot spots. It works for me but may not be suitable for everyone’s life style.

In the US, that option doesn't work since the SIM has to be "married" to the phone.