I was able to root it using GingerBreak.apk, pushed via adb.
But I think I discovered a way to do it without a computer:
1) Enable unknown sources.
Problem: The checkbox doesn't appear in Settings->Application->Development
Solution: Enter the secret menu (dial 3845#*506#); next to last is the option "Unknow Sources Test" (sic). There you can tick the Unknown Sources checkbox.
2) Download and install Gingerbreak 1.2 APK from
XDA.
3) Run Gingerbreak. After reboot, enjoy your rooted device.
Note that there still isn't a custom recovery for it, and the P500 is not fully compatible because of screen issues.