I have been thinking about this last week. First I thought it was some sort of "conspiracy", to make us all buy new data plans. However, the more I think about it, the more it makes sense: Apple just always goes with the newest and latest. They have changed their display ports about 4 times over the last few years, as new standards erupted, while the old would have sufficed.
Why not do that for sim cards?
Also, it's not really an issue, as you can just cut your current sim to fit the iPad. I'm sure as soon as the iPad gets any traction, there will be tools for this, as well as cheap plastic adapters to fit the uSim in your current phone.
I don't know if Monoprice ships to Europe, but there has to be a better alternative to the Apple Store there. You can get a comparable cable from Monoprice for about 8 Euros.