How do I unlock iPhone 4 running iOS 4.1?

If you can unlock or not, depends on the version of the baseband. To check the baseband version, open Settings, General, About, Modem Firmware. If the version is 1.59.00, you can unlock. If you upgraded to iOS4.1 without protecting your baseband, you have probably baseband 2.10.04 installed, which cannot be unlocked right now. In other words, you have to wait until a new unlock comes out.