Putting the phone into offline mode or removing the SIM card does turn the power off on the cellular radio.

But turning off the cellular radio won't actually affect battery life that much, all that a phone does in standby mode is occasionally send a signal to make the nearest cell aware it's connected. There's no permanent connection. The only time that the cellular radio really uses up the battery a lot is during calls or data access.

And on a device like the N900 with a relatively large 1300mAh battery the percentage of battery life you'd save from switching off the cellular radio is fairly small.

By the way, another option if you're just trying to reduce power is to switch to 2G (GSM) mode instead of 3G (WCDMA/UMTS). 2G modes usually have slightly longer standby times compared to 3G modes. The only disadvantages of 2G are the slower internet speeds and the inability to use the internet and call at the same time.