A great tip over on Gizmodo shows how iOS 4.1-toting iPhone owners can flip their handset into "Field Test" mode which shows the antenna signal strength (or more accurately signal to noise ratio) in dBm rather than as bars.
You need to be on the latest iOS 4.1 to do this trick. Then:
- Dial *3001#12345#* into your phone.
It's that easy. Tap on the numbers/bars to switch. To exit "Field Test" mode, hit the Home button.