To use DC-motors instead of stepper motors, you would need to have some sort of encoder that tells the controlling machine where the printhead has moved.
On the arduino Forums they have discussed the problems and difficulties. The main problem is, that currently, there is little firmware for Arduino or similar chips that can do this. Another big problem is, that such a machine might need a different type of G-code, unless well done.
That is not to say that there is no firmware that does control DC motors via an encoder. Printers do so since ages. But to reverse engineer printer firmware is really really hard. It took a pair of hackers the better part of a half year to dissect the printer & Fax firmware for one printer, and they had mainly looked to dissect the fax protocol and had not looked at all at the motor controlDefcon talk about it.
Luckily, as the RepRap tells though, somebody did hack together a DC motor control: Miguel Sanchez has a GitHub about it. You DO need proper encoders though.