I don't remember when it started, but I noticed my MacBook is running hotter these days, and the fan switches on to full speed after few minutes of login. iStat Pro, the dashboard widget, tells me that the process "airport" has very high CPU (about 99-100%). There is no application named airport in my system, so it's a child process of some other application.
After much tinkering around, I found the solution. The culprit was Vine Server, a VNC server application. I had deleted the application from my applications folder earlier, but apparently it left something behind. What did it leave? Spotlight didn't show any results for "VNC", but Quicksilver showed this:
So I deleted this app from my hard drive (I had to enter password), rebooted, and now everything works fine.
Lesson for Mac Users: If "airport" shows very high CPU usage, remove everything related to VNC.
Lesson for Apple: Please include a clean uninstaller with OS X.