I have a mim p bass with spb3's installed. When I touch the poles, I get a humming sound. If I touch the pole and a string at the same time with the same finger the hum is even louder. Anyone know what this is?

Not sure but it sounds like a bad ground somewhere.

Don't touch the poles.

Or is that Don't cross the streams?

Both good tips!

Did you install the pickups yourself? If you're like me (let's hope not) you may have left something ungrounded. That's what it sounds like to me, too. Take it to a repair person or open it up to see if you can track down what's supposed to be grounded and isn't. Good luck!

A lot of first timers see a grounding wire going to the bridge and think, "what the heck is this? I don't think it's even connected to anything." *snip*.

That's fairly common with P Bass pickups, either paint the pole pieces with clear nail polish or you can just cover the pole pieces with electrical tape.

Why do you touch the pole pieces?

Is the hum louder or quieter if you touch the pole pieces with your tongue?

If you do the above in the basement with bare feet, does it:
1) Hurt
2) knock you on your ass
3) kill you

If we don't hear back, I'll assume number 3 and you're problem is essentially solved, though not in the way you preferred.

Grounding. Ground wire from pickups and in some basses the bridge, should be connected to case of pots (not to mention one of the pins), as well as the sleeve of the 1/4" jack.