Can I use wifi on my iPhone 3G without a sim card?
Yes, this works fine.  Just use it as normal, so long as it is not in Airplane Mode it should be no different to having one in.  You may see the odd "no sim card" message at the lock screen, but just ignore it.

To test, I just removed my sim, I received a "no sim card" message (which you cannot "ok" to make it go away), but proceeded to unlock, open a webpage, re-lock and the message had gone.  Everything worked.