Can Pressflow 7 be a drop-in replacement for Drupal 7? The Pressflow site mentions that it is used by many Drupal 6 sites, and in the FAQ, in answer to the question "What's the deal with Pressflow 7?" says


  Pressflow 6 made significant improvements to Drupal core in
  scalability and performance and most of these features have been
  contributed back to Drupal 7 core. Until well understood performance
  patches are created for Drupal 7 Pressflow development will be slow.
  Additionally, as Drupal 8 matures, significant enhancements will
  likely be backported to Pressflow 7.


That sounds to me like a very indirect way of implying that there are lots of problems with Pressflow 7 that cause people to avoid using it, but stated in a way that no one would understand unless they're already aware of the problems.  The answer seems like more of an explanation (indirect and implied) of why there are problems, than really an explanation of what the situation is with Pressflow 7.  

So, can anyone tell me in plain English what the situation is?  Is there some reason I wouldn't want to replace a normal Drupal 7 site with Pressflow 7?  Or some reason it is simply impossible?  (Or is the problem simply that Pressflow 7 has not been optimized to the same degree that Pressflow 6 has?)
It's not that there are problems with Pressflow 7, but that there are no significant benefits to it at this time. This is stated in the lines above your quote:


  If your project is built on Drupal 7 you might not need to use
  Pressflow at this time because the key enhancements from Pressflow 6
  were added to Drupal 7.


You could use Pressflow 7 as a replacement, but why bother if there are no advantages? (I haven't actually used Pressflow 7 because of this).