I setup a Linode and ran the Mercury stackscript (ID=353).
Then I imported my /sites/default/files, sites/all/modules, sites/all/themes, sites/all/libraries, and DB from an existing D6.22 site.
Now I just get Varnish 500 error pages, and the logs show fatal PHP errors like this one:

Call to undefined function features_include_defaults() in /var/www/sites/all/modules/custom_formatters/custom_formatters.module on line 281.

But if I substitute my old /modules, and /includes directories and files, the site works.
I wasn't using the custom-formatters module so I just deleted it, but then the same error just moved to the Rules module, which I do use.

Call to undefined function rules_include() in /var/www/sites/all/modules/rules/rules_admin/rules_admin.sets.inc

All module directories and files in sites/all/modules appear to be normal. At first I assumed it was a file ownership/permissions error, but I checked that and set the site perms per usual. It's probably something stupid I'm overlooking.

Seems Pressflow isn't always a drop-in replacement for D6. To complicate matters, Mercury (and it's components like Pressflow) isn't exactly a community effort so there's no place for centralized support.

Any ideas what might be wrong?

I m having a drupal 6.22 site hosting here, and I'm considering to upgrade it to the Mercury profile.

Reading your post, gives me second thoughts.
Did you find a solution to this?
Is this a stackscript specific problem, or this is a Mercury problem?

No, sorry I didn't find a solution. I had a few posts on this at D.O and got zero response. Pantheon will be publicly available in about a month but I hear the starting price is ~$100/mo. So there will be demand to get this fixed eventually.

I'm not an expert by any means, but you must make sure you did not overwrite the modules installed in the sites/all/modules directory as the mercury script installs a few. Also, you need to add the varnish and memcache settings to your settings.php file.

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Color Genetics