Does using the iphone on a wifi network constitute as data usage?

Not with AT&T in the US. I assume it would be the same elsewhere in the world, as you wouldn't be using a 3G or cellular data network.

 

it shouldn't use any data. the "3G" logo in the top bar should be replaced with wifi bars.

also, you can go into settings > usage, write down the sent/received data numbers, use your phone a bit while connected to the wifi, and then check back. sent/received should be the same. that's what i did just to double check.

 

It does not constitute as data usage because with Wi-Fi, you're using the internet connection from your house/coffee shop/etc. rather than the 3G connection.

Here's one way to see for yourself (or to allow only Wi-Fi mode): go to the Settings on your phone, and turn on Airplane mode. This will turn off 3G AND Wi-Fi. Then go to Wi-Fi and turn Wi-Fi back on. Now your phone-related features and data won't work, but Wi-Fi will still work.

 

iOS 4 allows you to turn cellular data off. Airplane mode is also an option. With the data off, you won't be able to send/receive MMS.

 

This is really nice to know, actually the ATT guy at the store told me this when I bought my phone in June.
Then I dropped down to the $15 plan.
Thanks for bringing this up.