I've just installed the new version of P2 and it seems like there is hassle with posting images. When I click upload media button, I get 404 page in modal box.
When I check the url, it shows:
http://localhost/p2/p2/wp-admin/media-upload.php?p2-upload=true&post_id=0&type=image& (so the wp installation directory in URL looks to doubled (.../p2/p2/...)
After rollback to 1.2.2, everything is OK.
I'm pretty sure it has something to do with something from 1.2.3's changelog:
* Allow image upload from front-end regardless of domain and HTTPS setting
Just to FYI, running 1.2.2 smoothly :)