Just for posterity, I've since found that the pressure sensor in my pen was bad, so bad that pulling the tip out overnight didn't help. I've yet to buy a new pen though.. too expensive
Source, which I've
finally found, after how many years

:
http://www.wacom.com/faqs/view.php?id=379
EDIT:
There is a solution!
Utilising the Universal Low-Tech Gadget Fix method (aka "Hit the bloody thing"), I've managed to get my pen to work properly again!
1. Plug in your tablet, and remove the tip from your pen if possible (don't worry if you can't though).
2. Open the Tablet control panel in System Preferences.
3. Hold down Opt (or Alt, or ⌥) while clicking the "About" button to get to the diagnostic screen.
4. Hold the pen
near the tablet (not touching it) so it registers on the info pane. Check the "Pressure" reading. Mine was hovering around 50, some other people had it up around 220.
5. Actual solution: Bang the pen, side on, against something (the original solution called for a table top, I chickened out and used my thighs to lessen the risk of other damage to the pen). Try 10 hits for starters.
6. Hold the pen to the tablet again. The "Pressure" reading should have drop a little. Sometimes it might not change, but be patient!
7. Repeat steps 5 and 6. Eventually the stuck pressure sensor would loosen, the pressure drop to 0, and the tablet will work properly again. Remember to replace your pen tip if you removed it in step 1.
I hit mine a total of maybe 100 times, gradually using more force at the start until the "Pressure" reading started dropping, then using a similar amount of force thereafter. The pressure dropped slowly from 50 down to 25, then suddenly back to 0, so check the readings often so you don't hit the pen more times than necessary. It
is a rather delicate gadget, after all!
If your sensor's stuck under 60 (eg, 50 for mine), and you don't want to hit your pen/can't drop it to zero, you can adjust the "Pressure offset" from 0 to 59; put it to something just above your stuck sensor value (eg, I'd put it to 55 for mine), then you can at least use the tablet, albeit with a slightly reduced pressure sensitivity range.
Disclaimer: YMMV, and this method probably won't work (in fact, will probably make it worse) if your pen actually has a broken magnet/coil/circuit board rather than a stuck pressure sensor. I'm not responsible if you break some other part of your tablet/pen/computer/table/thighs while attempting the repair. If it's under warranty, best to return it to Wacom
Source:
http://www.fixya.com/support/t1555723-wacom_graphire_4_stylus_malfunctioning (scroll down to "Best solution (1 of 3)")