1) Bluetooth operates in the 2.4 Ghz radio frequency range.
2) 802.11b/g WiFi operates in the 2.4 Ghz radio frequency range.
Despite all of the reassurances by the Bluetooth special interests and manufacturers that BT and 802.11b/g WiFi will not interfere with each other, it appears that you're holding proof to the contrary right in the palm (not to be confused with Palm)

of your hand.
Since BT is the lower-power, more time-critical protocol, it makes sense that it would choke first with an active WiFi transmitter in the phone. I have personally observed my own Pre refusing to sync to my BT car kit in the parking lot at work if I leave the WiFi interface on and it has a WiFi link to the office network, and am guessing that you may be seeing the same problem.....