Maybe instead of just adding a readme.txt tag, we should also encourage putting the up-for-adoption Plugin on GitHub (if it's not already there), so that anyone wanting to adopt the Plugin can make pull requests?

That's basically what George Stephanis did with Update Control, which I now maintain. The Plugin was already on GitHub, and I started making pull requests. He then added me as a GitHub committer, and then as an SVN committer.

The problem with *just* going with the "adopt-me" tag is that the only way to implement an adoption is to skip straight to the SVN-committer step, which requires a great deal of trust. But with GitHub, there's a built-in trust-building mechanism:

1) Pull Requests (merges must be accepted by the repo owner)
2) Add as GitHub committer (can commit directly to GitHub repo)
3) Add as SVN committer (can commit directly to Plugins SVN)

Note also: this mechanism doesn't even require an "adopt-me" tag; anyone who finds a Plugin that could use some love (a bugfix, major updates, whatever) can make pull requests if the Plugin has a GitHub repo.

Side thought: how could would it be to have git.wordpress.org/chipbennett/plugin-slug - basically, a WordPress-centric GitHub?