If I access a website and complete an online form using 3G on my iphone, can my actual phone be traced back to the IP address? For example if the website can see what IP address was used, how easy is it then to match the IP address to my IMEI
No, the website cannot, but the phone company can. They will know which account was using the IP and what IMEI is used with that account.
While I'm not sure about the IMEI it might be possible to track the phone number or other information associated with the device because some mobile provider insert these information inside a HTTP request. See the paper Privacy Leaks in Mobile Phone Internet Access for more information or a summary and interview with the author in Why are websites getting your mobile-phone number?. See also the post Detect an MSISDN (mobile number) with the browser.
The IMEI is used as an id for you mobile in the GSM NEtwork,the GSM network here is the radio link betwen you and the GSM antenna providing the mobile network cover and the back bound which is totally hidden for you and for the website that you are trying to reach.
when you start the mobile phone it broadcasts to you mobile phone operator to identify valid devices. it can be used to stop a stolen device or tracking a target, but all this happen at you mobile network operator/ Gov level.
Visiting a website can't unmask you IMEI unless you provided it or a 3ed party software was used to revile it. Website can't know if you are using wifi or 3G or if you are using a mobile phone to connect unless there are some indications:to detect the clients, some services use the screens size, the web browser, IP range ... etc to identify mobile users from the rest of visitor.
So to answer your question, this is not possible.
They will trace back the ip to the ISP, the isp will have information that will allow him to correlate the ip with a network session and a session with a contract (SIM CARD / IMEI) / person.
The contract will have your information. In Europe all mobile phones / data cards need to be associated with a person since some years back. That does not mean that they are efficient collecting that data...
I do not know who it is in the rest of the world. But assuming that you have an anonymous connection the ISP will not be able to associate the ip with a contract or person but they can still do it to the hardware. IMEI is the hardware identifier that together with your SIM card provide a unique session id to your ISP.
The IMEI can be changed that will make it harder to trace back and if the SIM card is obtained in a total anonymous way then it will be very hard to put a face associated with the ip...
Specially if you rotate the IMEI and SIM card frequently.