That's right. The 2G connections (GPRS and EDGE) use the same GSM technology and wavelength as phone calls, so you can only use one at a time, either data or voice, but not both.
However, I think the problem being complained of is that while the N900 is using data via GPRS or EDGE and the call comes in, the device does not automatically disconnect from the data connection and handle the phone call. I've only used iPhone 2G with data enabled and whenever I'm using EDGE and somebody calls, the data connection drops to allow the phone to handle voice. I haven't experienced this with my N900, so I can't tell whether this affects all devices.