If your warranty is expired or you don't mind voiding it, I think I've finally found a solution to this problem.
If you take the stylus apart, you'll see the nib/pressure sensor/iron core and coil assembly at both ends of the fragile board. They're both made of six small pieces: The two large grey and tan halves of the assembly, a white Nib holder, a rubber pad, an extremely thin spacer, and some sort of magical disk thing. this assembly can be opened by gently slipping a fingernail into the small gap around the lip of the grey half, where the two meet, and carefully working the clips free. All of the small parts inside are unsecured, and will probably fall out, so try to separate the two halves slowly.
What I found is the black rubber pad starts to bend inward in the same shape as the rounded half of the Nib bracket, by just a few thousandths of an inch. This is what is triggering the pressure sensor even without a Nib in the pen.
Flip it over so the side developing a bump is now towards the white Nib holder, with the gasket between that disk shaped thing and the rubber pad. Put it all back together, and with any luck, it'll start working like new again.
When I first got the assembly apart, that magic disk was flung out. When you go to replace it, the single metal face was towards the inside of the tan mount, facing the brass contact surface. However, it doesn't seem to matter which way I put it back in, oddly enough.
There's a good walkthrough on how to disassemble the stylus at: http://wishidknownthat.blogspot.com/2009/04/fixing-wacom-bamboo-pen.html
The sensor assembly is clearly visible in the eighth image down.
Hope that helps!