Will constantly having Bluetooth on drain my battery too fast? I'm thinking about making an app which will require Bluetooth to be available at all times.  However, if this reduces battery charge life significantly, then that's kind of a show stopper.  Can anyone give me a general idea about how much drain constant Bluetooth use will put on the battery?
What drains... my special. Here some data samples, based on a Motorola Droid:

+--------------------+----------+
| Mode               | Energy   |
+--------------------+----------+
| Bluetooth receive  | 751 mW   |
| Bluetooth send     | 487 mW   |
| Bluetooth standby  |   2,8 mW |
+--------------------+----------+


So if you use BT quite regularly (e.g. to be able to immediately pick up a call with your headset), having it in standby doesn't hurt that much (Compare: 2G StandBy 11,6 mW, 3G StandBy 18,3 mW, Wifi StandBy 7,8 mW -- and, often mistaken: GPS StandBy 0,4 mW).

But if, on the other hand, you use BT once a week (or even less) -- heck, you can manually enable it then, there are enough "toggle switches" and widgets available.