You don't have 3G data service with AT&T and your N900, only 2G or 2.5G (Edge) service. No data + simultaneous calling without 3G.

Verizon customers even with 3G service have the same problem - no data service while making calls. It's a limitation of the CDMA system.

If you go to T-Mobile 3G service you can surf + call in areas where 3G service is available.