This will no doubt be because the amount of data transfered to view webpages on the N900. Maemo will display webpages as they are supposed to be viewed including ALL of the flash content and whatever else they may have, where as many other nokia/symbian phone will be redirected to the cut down flash-free Mobile versions of a companies webpage.

Downloading Apps through 3g connection will also add to the cost too as Maemo apps tend to be larger than their Symbian counter parts.

Remember, the N900 is an Internet Tablet first and a phone second. You may want to look into getting an unlimitted dataplan.