You just need the Linux version of Unrevoked3.21 from their IRC channel. The program needs root access on the computer to communicate with the phone correctly. This Ubuntu forum post will show you how to use Terminal to create a drag and drop launcher that you can make that will allow Reflash to run correctly.
HOWTO: Easily open any file as root via drag & drop - Ubuntu Forums
Worked perfectly for me!
Found an easier way to create the root launcher.
1. Right click on the desktop and click "Create Launcher"
2. In the name section put something like "Root Launcher" or "Root"
- in the command line put [gksudo "gnome-open %u"] without brackets
- you can leave comment empty
3. Now drag and drop the Reflash package over this icon
4. Now double click on Reflash and Unrevoked should start running
Good Luck! Let me know if you have any problems or questions.