How to hide User Id's in Privatemsg
I don't think there's an easy way to replace the user IDs with usernames, but you can easily add the usernames using the Realname integration (you have to set up the Realname module as well though).

My Privatemsg send page looks like this:



I did this by enabling Realname and following the instructions here.  You will need the dev version as well; the 1.3 release has broken Realname AJAX.