Just a random thought: as a German I have the impression that people here would be much too paranoid to do something like that over the internet, even with SSL.
The German variant is aptly called "Elster" and while it uses ssl it lacks authentication. I could file a bogus tax return for you if I know your tax number. It's also mandatory for businesses when you need to file a VAT statement. Oh, and last time I checked, the client software required windows - there's a less comfortable online version though.
You can file a bogus tax return for someone in the US just as easily. I'd be surprised if that weren't the case in most countries. I believe there have been occasional incidents of people doing so to collect other peoples' tax refunds, but I don't hear about it very often.
VAT statements in Germany are a bit riskier though: If you give the Revenue Service permission to withdraw money and I file a VAT statement for you which shows a large income they'll just go and collect 20%. It's a bit an edge case but that's the reason I manually send the VAT collected each month. I'd feel much safer if the interface was authenticated in some way.
