So here's my port for D6. I was hoping to test this on a production authorize.net account rather than just the developers test account, but maybe someone else can test it with theirs. I would love to hear how it's working if you do.
Besides that, everything seems to be working well for me so far. I believe I've tackled a couple bugs and even got a new feature or two in. If this turns into an official release I'll come up with the changes I've made for an official change log, but for now I'm just gonna say, try it out!

I'll fix any bugs that you guys discover so please be post them here. Chad - I know this is sort of an unorthodox way to approach co-maintaining, but since I need to use this module for a project, I'm gung ho about getting it ready to go. Let me know if you feel its necessary to coordinate more closely than this.