I saw this same issue today after upgrading to WordPress 3.5 on a blog we're developing right now (not yet in production). Oddly, we had another copy of the same blog hosted on the same server which we upgraded to 3.5 first, and it worked fine. I compared the file contents of the two blogs and they were identical, yet one reported this php error:
PHP Fatal error: Cannot redeclare get_post_to_edit() (previously declared in <blogroot>\wp-admin\includes\post.php:473) in <blogroot>\wp-admin\includes\deprecated.php on line 993)
This made me suspicious of state, so I reset the IIS server that was hosting both blogs, and the problem went away. This makes me suspect that perhaps some old php content was cached (which I have seen before in IIS and is maddening to diagnose). This theory is further supported by the fact that line 473 of post.php (as listed in the php error) is in the middle of a comment block -- it's not a function declaration. It looks like the get_post_to_edit() function was deprecated in the 3.5 release and moved to the deprecated.php file, but the cached copy of the old post.php still had it declared.
Not sure if you have access to your IIS server, but if you do, running "iisreset" (from a Run As Administrator command prompt) should fix the problem. I'm also interested to hear from others hosting wordpress on IIS (not my choice) and how you deal with this php caching issue.