In addition to Thomas's excellent advice in removing the current incarnation of the MacDefender Trojan, one should also take into careful consideration that malware evolves and is altered and delivered by other parties.
What steps you are taking following Thomas's may work and appear to be enough, but it's impossible to be 100% sure as you can't compare his version of the malware with the version you have.
My advice is to take Thomas's advice as a first step, then take a additional measures to backup your files and resintall the operating system from the (hold c bootable) OS X installer disks after using Disk Utility to Zero erase (under the menu) your boot drive (all data will be destroyed, format HFS+ Journaled) and then reinstall OS X. Re-install programs from fresh sources.
Yes, it's a lot of work unfortunatly, if you don't know how to do this, take it to a computer professional who can.
If you didn't give this Trojan (or any malware) your administrative password (or it didn't gain root access some other way), then my steps above are not necessary.
To prevent this MacDefender Trojan from happening again:
It preys upon a JavaScript vulnerability on web pages among other things.
Since turning Safari's JavaScript Preference on/off constantly is a chore.
I advise using the Firefox web browser and the Add-On: NoScript which in Firefox Toolbar customization you drag a Noscript button to the toolbar or easy on/off of all scripts and plug-ins.
NoScript also offers other "web cop" features, it takes some getting used too as your surfing the web without anything running, then turning it on per site basis once you trust the site.
Firefox also has a download opt out window before it downloads, giving you a chance to stop this thing in it's tracks.