Here is the process I just used for exporting a pull request to GitHub.  It went incredibly smoothly.  I will put this in a migration document as well.

Step 0: Make sure you have hg-git installed (it is pip installable) and enabled in your ~/.hgrc .  Enabling it looks like this:

[extensions]

hggit=

Now, here are the steps:

 1. Fork http://github.com/yt-project/yt

 2. In your yt source repository, update to the latest changeset of your pull request.

 3. If it does not have a bookmark, create one with "hg bookmark". It can be something like "hg bookmark whatever_i_want_to_call_it"

 4. Edit your .hg/hgrc in your yt repository.  Under [paths] add a github one.  For me, it looks like this:

[paths]

default = ssh://hg@bitbucket.org/MatthewTurk/yt

ytanalysis = ssh://hg@bitbucket.org/yt_analysis/yt

gh = git@github.com:MatthewTurk/yt

ghyt = git@github.com:yt-project/yt

You will need to modify `gh` here.

 5. Now, check what you're going to export.  "hg outgoing -r whatever_i_want_to_call_it gh" will show you.  Make sure that this matches what you anticipate.

 6. If it all looks good, push it *with the bookmark*.  "hg push -B whatever_i_want_to_call_it gh" will do this.

 7. Reopen the pull request on GitHub, and decline your pull request on BitBucket.

Here is the one I just did: https://github.com/yt-project/yt/pull/1346

This should make things very smooth to transition.  Since we're not yet *accepting* things on GitHub, I would encourage folks to start moving their long-term PRs.  (does that sound right, Nathan and Kacper?)

-Matt