Thanks for making up the checklist. For starters, let me just inject something at the top of the list and then interact with it.
I think the absolute first thing to happen should be the 1.0 release. Porting beta code to D6 that we'll have to then fix in two places just doesn't make sense. Also, spending our development time now on updating to D6 means the 1.0 would get delayed even further. One of the questions from "The Joel Test" that I think bites a lot of module developers is, "Do you fix bugs before writing new code?" I'm not saying UC should be perfect (because I know it's pock marks all too well), but we should definitely solve known bugs first.
That's fine with me... are you thinking just tracking all the updates in one mondo patch? That'll get ridiculous pretty quick I suppose. Maybe starting off w/ just the 4 core modules will be the way to do it.
Converting 5.x modules to 6.x:
http://drupal.org/node/114774
I've done this off and on and worked on a little budget module for my wife and I in D6, and what I've seen makes me think there are going to have to be some over-arching design goals for how to use Ubercart w/ the new menu system especially. Particularly I'm thinking the use of specifying the "file" attribute for menu paths and also using this trick I picked up from chx in appropriate places to trim down our menu sizes.
Third: check dependancies:
Workflow-ng- Tables API
- Token (dev snapshot works)
- uBrowser
of these that aren't upgraded, suggest working on updating these first. Or finding D6 replacements
This'll be a biggie, b/c we do rely on several other modules for core image support. That's a must have feature for Ubercart, so we will be a little at their mercy.
Fourth: find any fundamental code changes in ubercart, caused by drupal6
example: all the postgres install stuff, the install process in D6 eliminates this problem. Note: still need to make sure our queries are Postgres compatible..
Agreed. New menu system, updated form system, new schema API... Ubercart is a set of pages, forms, and install files. Essentially everything we've done will be affected. 
D6 actions -- some of the folks here I've talked to hate workflow-ng with a passion. I think with the upgrade to 6 we should nip workflow-ng in the bud and use actions.
http://drupal.org/node/172152 --> actions
I still disagree with this primarily because there are no conditions. The beauty of Workflow-ng is that it has a UI (even if it's a little rough) that allows folks to attach actions to events based on conditions. We simply have to have them for Ubercart. If the core actions module supports this, sign me up. Otherwise we'll have to look into porting Workflow-ng or trimming it down and adapting it for core Ubercart. Right now, Workflow-ng handles conditional taxes, shipping, order updates, etc. And we hope to outsource notifications to it when we update and drop the uc_notify.module altogether. Such a core module can't just be abandoned without almost a 1:1 alternative.
Fifth: with all these changes, obviously ubercart will become branched.. which then means back-porting changes, etc... The only way a D6 conversion will work is if we have people working in both branches to keep them synced, and ugh this won't be easy to do.
Yeah, I'm not too sold on the idea of maintaining the 5.x version indefinitely. At least the core team isn't planning on spending a lot of time maintaining a branch for what we'll see as an inferior version of Drupal. It just makes sense for the software to move on with the platform it's built on. I'm not saying we'll abandon it, and we'll definitely port back major bug fixes... but new features can be counted out without an easy drop in patch and a good explanation for inclusion. 
Will be interested to hear your response on these things.
Joined: 08/07/2007