During December and January I had the occasion to visit three countries in which I wanted to use my unlocked Huawei E220 UMTS dongle for prepaid UMTS ("3G") Internet. Here are my experiences, in brief.
Australia was the only real success of my trip. Although it takes about 20 minutes to buy a SIM card in Australia, it is not expensive (around AU$10). I bought two cards for data, one of which I could net get to work, but the second of which - from 3 Australia - worked just fine. Data usage is not expensive - a $20 voucher bought me 2GB of traffic. Another nice experience in Australia was my hotel in Melbourne, which offered - again, inexpensive - prepaid ADSL usernames and passwords and a connection to an DSL access point over the hotel's Ethernet network.
The reason it takes long to buy a SIM card in Australia is the requirement that one should fill in a standard registration form - even for prepaid - from the Australian federal government. Needless to say, I ended up "accidentally" giving incorrect details all three times (once for voice and twice for data) that I purchased a SIM card and it seems that two different credit cards are considered sufficient verification of identity for these purposes.
In New Zealand, it was very easy to buy a SIM card for voice but at NZ$35 it was a bit expensive. I could not work out how to activate it for data and I was unwilling to purchase another one for further experimentation. If I remember correctly, there was no paperwork involved. Most of the places where I stayed, all at the lower end of the market, did offer free or inexpensive Internet for guests. New Zealand has the curious situation where there are really just two cellphone networks, using incompatible technologies (CDMA and GSM), so the competition is somewhat restricted.
The UK was, in some sense, the most frustrating experience of all. I had purchased a SIM card from T-Mobile over the Internet before going and after working out how to activate and recharge it, it worked just fine for voice. A nice feature about the UK card was that it had a different magnetic card in the starter pack that can be swiped in many stores to identify the number when one adds credit. This dispenses with the entering of silly voucher numbers etc.
In spite of the fact that one sees wireless data plans advertised widely in the UK, at very attractive prices, I was unable to find anyone prepared to sell me a SIM card to work with my own modem. All the shops that I visited, claimed that it was necessary to buy a modem as well - whether on a prepaid or on a contract service. The supposed 3G service on the SIM card is supposed to work for WAP browsing on the phone only, although it is not clear to me whether it is technically possible to limit it, and I could not even get that working, anyway. One of the best experiences was visiting an Orange shop in the large and fashionable Bullring centre in Birmingham, full of phones and people, where I asked to buy a SIM card after the shop assistant (accurately) reported that she did know whether it could work. She came back after 5 minutes to tell me that they "did not have SIM cards in stock". I know when to give up.