[HTC Inspire 4G] henrybravo:
Another unscientific method that will give you a decent indication of determining just the raw idle cell radio draw is:

download and install Battery Monitor Widget (it's a free download on the Market)

disable GPS, bluetooth and wifi

disable background data and auto sync

disable any type of automatic data polling or push notifications in your apps (email, twitter, facebook, etc)

reboot your phone so that you can perform testing on a freshly loaded OS with minimal apps running.

With your phone in standby mode (screen off), let the Battery Monitor Widget monitor your battery drain for an hour or two, or longer if you've got the time. Take note of the time your phone entered standby mode. BMW displays the milliamp draw during each of its polling cycles. Here is a sample of what the polling looks like (I've got BMW polling every 3 minutes):

2011/06/10 | 02:23:25 PM | -5mA | 79% | 3974mV | 22.0°C

2011/06/10 | 02:20:25 PM | -1mA | 79% | 3974mV | 22.0°C

2011/06/10 | 02:17:25 PM | -3mA | 79% | 3974mV | 22.0°C

2011/06/10 | 02:14:25 PM | -3mA | 79% | 3974mV | 22.0°C

2011/06/10 | 02:11:25 PM | -5mA | 79% | 3974mV | 22.0°C

2011/06/10 | 02:08:25 PM | -11mA | 79% | 3974mV | 22.0°C

2011/06/10 | 02:05:25 PM | -5mA | 79% | 3974mV | 22.0°C

2011/06/10 | 02:02:25 PM | -1mA | 79% | 3974mV | 22.0°C

Observe the 3rd column; this is the milliamp drain of your battery. Now scroll down and observe all the milliamp numbers, but only for the time period your phone was in standby mode. The average of these numbers is your battery drain in standby mode with your cell radio ON.

Now put your phone into AIRPLANE mode. This will completely shut down down all the radios in your phone. Since you already had your GPS, bluetooth and wifi radios disabled from step #2 above, putting your phone into airplane mode will only additionally disable the cell radio. Now let your phone just sit in standby mode for a while and observe the battery draw. Unless you've got an app running in the background that is using your battery, in airplane mode your milliamp numbers should be -1 mA every poll, and look something like this:

2011/06/10 | 02:46:37 PM | -1mA | 78% | 3967mV | 22.0°C

2011/06/10 | 02:43:37 PM | -1mA | 78% | 3967mV | 22.0°C

2011/06/10 | 02:40:37 PM | -1mA | 78% | 3967mV | 22.0°C

2011/06/10 | 02:37:37 PM | -1mA | 78% | 3967mV | 22.0°C

2011/06/10 | 02:34:37 PM | -1mA | 78% | 3967mV | 22.0°C

2011/06/10 | 02:31:37 PM | -1mA | 78% | 3967mV | 22.0°C

2011/06/10 | 02:28:37 PM | -1mA | 78% | 3967mV | 22.0°C

2011/06/10 | 02:25:37 PM | -1mA | 78% | 3967mV | 22.0°C

The difference in these milliamp averages between airplane mode on and off is a good approximation of your idle cell radio battery draw.

One thing I have found in my testing is that when my phone is stationary in a decent signal area, my cell radio draw is minimal (like in my example above where the draw is averaging around 4 or 5 milliamps). However when I'm driving or walking around, the cell radio drains quite a bit more juice (averages around 5-20 mA with brief spikes to 150mA). Naturally the assumption is the cell radio uses more juice as it tries to hang on to weaker signals and as the signal jumps from tower to tower as you're traveling around. I've been testing different radios and RILs to see if I can get an improvement in the "traveling around" cell radio battery drain but haven't come up with anything yet that makes a significant difference.