What's the best way to contribute back stacks and packages?

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Should we still try to publish this to the community

Yes!

What would be the best way to publish?

Whatever way is best for you, it's your code. To avoid having to set up and maintain your own external facing servers there are lots of options. Having tried a number I ultimately settled on hg hosted on bitbucket and I've been happy with that, but YMMV. You'll have to pick what works best for you. Other options are git+github, svn/hg+googlecode, bzr+launchpad and I'm sure many more. The ros indexers seem to play nice with all the above mentioned VCS's which is nice.

Merging kwc's answers too:

http://www.ros.org/wiki/Get%20Involved

As for the rest of the bullets:

* Is there a preferred or ROS official VCS host that we should/could be using?

We prefer hosts with the best performance and uptimes, which are probably the same criteria you would use as well.

* What are the licence requirements/guidelines? BSD/GPL/Mozilla/Apache?

BSD and Apache are generally the friendliest licenses if you wish to encourage use. We have had to do additional work on occasion to remove GPL code from libraries.

* Should we just edit the wiki to include links to our stuff, or is that frowned upon?

You are welcome to create wiki pages for your own packages/stacks. You should use discretion when editing wiki pages that aren't yours.