You don’t have to pay for a data plan. I use one of those prepaid 3G USB sticks and pay 2.50€ for one day unlimited HSDPA internet access (limited to GPRS speeds after 500MB).
That’s perfect for me since I’m maybe six hours in a train two or three times a month and no flatrate can beat the five or eight Euro I’m paying.
There are dozens of other options available (at least in Germany), suitable for pretty much any conceivable need and some will come with extra SIM cards for your smartphone or tablet.
How about this one: 15€ per month, unlimited HSDPA internet access (limited to GPRS speeds after 1GB), 9ct per minute for all phone calls and SMS (as always in Europe, incoming phone calls and SMS are free), two SIM cards. You could order an unlocked iPhone from Apple right now (I don’t know whether that’s possible in the US but it is now in Germany) and put one SIM card in the phone and use the other one for your laptop or whatever other device.