Reflashing won't help. It is not a problem, actually, it is just the way it works: n900 is designed to keep persistent data connection, but, unfortunately, the real problem is HSPA does drain battery very quick. You may use "autodisconnect" program to work that around. There is active connection indicator (looking at cellular data network icon is wrong place to find out if data connection is active, it just shows if one is _available_), it looks like two arrows in round shape for cellular networks.