Unfortunately, the solutions to this seem quite limited.
Droam50 will provide you with 50MB of data per day €27.50 for the first day and €3.50 per day afterwards. Alternatively, you can receive 1GB for €65 and €45 per GB (or part thereof!) over 30 days. Not very flexible at all. Various countries have certain limitations - but it works quite well across Europe. Unfortunately, you also have to pay expensive shipping costs for receiving and returning a Droam. In Australia, it would cost me €68 just in shipping costs.
Another option seems to be to obtain a pre-paid SIM in Europe and add a booster. T-mobile has boosters that cost between £1 for 3MB for a day, or £5 for 20MB over a week, or £10 for 50MB over 30 days. You may need to pay for some normal credit as well. You may also want to add a booster for normal calls.