I have just finished implementing, backporting, and enabling basic support for transparently using content from the Wikimedia Commons on all Wikimedia wikis.
An example for this: http://commons.wikimedia.org/wiki/Image:Onopordum_bracteatum0.jpg => http://en.wikipedia.org/wiki/Image:Onopordum_bracteatum0.jpg => http://de.wikipedia.org/wiki/Bild:Onopordum_bracteatum0.jpg => http://meta.wikimedia.org/wiki/Image:Onopordum_bracteatum0.jpg => http://ja.wikibooks.org/wiki/%E7%94%BB%E5%83%8F:Onopordum_bracteatum0.jpg
etc. etc.
Use a file from the Commons like you would any other, using [[Image:]] or [[Media:]] links, with extended options or without. If the file does not exist locally, we search the Commons. If it doesn't exist there either, we show a "Missing image" notice.
You can edit Commons image pages, but this should only be done for the purpose of describing the content. For licensing information, you can use MediaWiki:Sharedupload to insert an automatic backlink to the Commons. See the version on en: as an example: http://en.wikipedia.org/wiki/MediaWiki:Sharedupload
This is not perfect yet. In the not so far future, the content of the Commons description page should be directly transcluded on the using wiki. It should also be possible to upload content directly to the Commons from anywhere welse. In order to deal with issues like cross-wiki authentication, and to have a shared database in place, I am waiting for the single login transition before tackling this issue further.
And even these advanced features are just a mid term perspective. In the long term, I would like it to be possible for any open content wiki using our software to immediately start using Commons content (caching the images, of course, and maybe even serving them as mirrors chosen in a round-robin fashion).
In the meantime, please do start using the Commons, upload images and sound files to it, and help organizing the growing number of them. Keep in mind that the Commons is *only* for free content -- no fair use allowed. Having to upload non-free images locally is a good disincentive, and helps us to enforce individual wiki policies on non-free content.
All best,
Erik