I've been talking to the lullabot folks, as well as some of the classmates here in LA, and there are quite a few things that will need to be done in order to make ubercart D6 compatible.

First, I suggest we probably put this in the issue tracker. I have a stock D6 cvs download that I'm working from, and I'll start working on ubercart next week.

Secondly .. ANYONE who wants to work on this project needs to first study and understand the following:
Converting 5.x modules to 6.x:
http://drupal.org/node/114774

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

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..
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

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.

My company has pretty much been sold on the awesomeness of D6, and don't like the thought of building a site that works for D5 then having to upgrade again to get new features... It sounds like a few others are in this boat, so I think we can do it!