I was having the same issue upon installation on my M4400. I expected it as the RC note on teh page stated there were upstream kernel and driver issues with b43 based cards.
Here is what I did.
Installed LM13_KDE (RC) from teh DVD using a wired connection.
After rebooting into the new system I did ALL the updates and selected the backports as I always do in the sources manager.
Then updated again. then installed (using the software manager) the firmware-b43-lpphy-installer, which also in turn installs the b43 fw cutter.
Reboot. Then all worked for as you said about 5 minutes. dmesg revealed the failure of the firmware and unable to reinitialize the card again. I then used the hardware switch on the dell machine to disable the wireless card, and re-enable it and it started again.
The kernel update the next day solved the issue and my wireless now works without hiccup.
(broke my bluetooth, but that is another issue)
Hope this helps a little...