iPhone tethering uses cell data only, even if wifi available

Your iPhone can connect to the Internet through two main channels: wifi and cellular data (3G or if that is not available or disabled, EDGE).  However, I found that when using Internet Tethering, all packets to and from the tethered device appear to go through the cellular network ONLY, even if the iPhone has an active wifi connection.

(You may ask why you would use tethering on the iPhone when you have wifi available that you could use direcly on your laptop, but that's not the point.)

On the iPhone itself, connections still go through wifi.

Also it is not at all clear to me whether Internet Tethering data usage is showing up in General->Usage->Cellular Network Data.  I've tried doing some big downloads on my laptop and the Cellular Network Data usage numbers don't budge.

What this means mainly is that if you're doing Internet Tethering, you might as well turn wifi off on the iPhone, because you're not getting any benefit from it and it's using battery (as well as the same frequencies that Bluetooth uses).  I tried to see if tethering over Bluetooth was any faster with wifi disabled on the iPhone, but I didn't see any consistant pattern.  These are some of the speed tests with my PowerBook G4, iPhone 3GS Bluetooth tethering, Rogers Canada, downtown Ottawa:

wifi on

wifi on

wifi off

wifi off

Comments

[RM]

Any solutions?