Do not confuse this with the original, and entirely legitimate, MacDefender (http://www.macdefender.org/index.html)
As of today's date there are no known removal processes available. A fix should be posted on the Mac forums within the next couple of days.
Go into the Applications Folder > Choose MacDefender.app and remove it. If you can't delete it, open Activity Monitor and close the MacDefender and try again. You can do the same in safe mode http://support.apple.com/kb/HT1455
McAfee will provide cover against this threat with the download of DAT 6336.
Threat Identifier(s): FakeAlert-Macdefend, MACDefender
Threat Type: Malware
Risk Assessment: Low
Main Threat Vectors: Web; LAN; WAN; Peer-to-Peer Networks; E-Mail
User Interaction Required: Yes
Description: The FakeAlert-Macdefend Trojan masquerades as a legitimate security software package for Mac OS X. Similar to Windows-based counterparts in this family, this malware installs via a drive-by download, reports false security warnings, and presents suggestions to "clean" the system. This process involves paying for a "cleaner" or "more robust" version of the software. When running, the Trojan will display intermittent, false malware detection warnings, and open browser windows to pornographic sites.
Importance: Low. This threat has gained media attention.
McAfee Product Coverage *
DAT files: Coverage will be provided as "FakeAlert-Macdefend" in the 6336 DATs, to be released May 4.
McAfee VIL: FakeAlert-Macdefend.
SANS: More on MAC OSX Malware - MACDefender Fake Antivirus.