Diecimila and external BT module

Hello all. I've done a search around and haven't found anyone asking this question before, but feel free to point me in the right direction if indeed this has previously been covered. Basically, I have my Diecimila connected to a separate bluetooth module which is supposed to be acting as a transparent serial 'relay'. It is configured to use SPP, it is connected via bluetooth to my PC, and it is hooked up to the Arduino via the Tx and Rx pins. Using the serial monitor built into the Arduino IDE, with the relevant 'virtual' serial port selected, I can happily receive data that the Arduino is sending me via Serial.print()s. I would assume I can also send data in the other direction, although I have yet to test this.

My problem is as follows - despite this seemingly functional serial-over-bluetooth link, if I attempt to upload a sketch using the IDE, the software hangs during its 'uploading' phase.. and then vanishes into thin air after about a minute or so. Poof. I am guessing there is some quirk in the way that bluetooth/SPP functions which makes it work slightly differently to a standard serial link, and either the Arduino or the IDE doesn't like this. I understand that the Arduino BT supports uploads via bluetooth; is there anything special they have done (and thus that I can do) to it to make this work?

May be of note - I still have my Arduino plugged into my PC via USB in order to provide power, and so the jumper is set to keep the USB enabled. Would this make a difference; should I unplug it and use an external power supply instead?

Apologies for the epic post, any help would be much appreciated!