Not exactly the most useful hint, but I figured someone may want to know.

If you need to check the Apple Bluetooth software version, holding the Option key while clicking on the Bluetooth menu bar item will display the version number in the first (grayed out) row, right next to the On/Off message.

[robg adds: If you want even more details about the adapter, try clicking next to Bluetooth Device Name in the Bluetooth System Preferences panel, as explained in this hint.]