Can I use a factory unlocked iPhone 4 without a plan?

I want to buy a factory unlocked iPhone from amazon or eBay and not activate a plan with it (aka no texting or calls) would it work? My friend got one from her friend, but hasn't activated it yet but can still use it. I want to be able to use everything (apps, camera, etc.) but just not have an actual phone plan. Can I do that? And to my understanding if its not unlocked then i would have to actually have a plan to use it. And if it's unlocked, would i need to buy a SIM card to use with it? Any information on the topic you have would be great, i just don't want to buy the phone for about $500 then not be able to unlock it or use it or anything. Thanks so much for any information!

You could do that, but the whole reason why the iPhone costs so much is because it has a radio inside it that can perform calls and receive data. If you're not going to do that I don't suggest you get a phone.

OK. Good question!
I have friends that have used their iPhones for exactly what you are talking about. It's actually a really good idea. It's a very good camera and video camera. It has superb recording and playback quality. You can load up aps, play music and video and not have any phone plan!
One other thing that may not be known is that you can use it to surf the web when there is a WI-FI network available like at McDonald's or Starbucks or at home.
You won't need a sim card to do this either. You can activate the phone over a WI-FI connection and get rolling! No service plan required. Activate does not mean through a wireless carrier btw. You are simply setting up then phone for use!
Like you said you just can't make calls or send texts.

Yes, but why not get an iPod Touch. Will save you quite a bit of money.