I've been having this issue with my workstations for about two months now.
It doesn't seem to be triggered by anything I can find. At first I thought
it was the version of the iPrint client I was using. All of my users right
now are on 4.12. I started seeing this problem when we upgraded to 4.11 but
not everywhere. For those having the problem I back-reved there client to
4.05 and that seemed to fix this issue, for a while. Then it came right
back. Once 4.12 came out I tested it, felt that it had solved my problems
and pushed it out to my users. Everything was running fine for a while
again and certain users started having the problem of application going "Not
Responding" again. One side step (work around) I have found to this is to
install the same printer with the same driver to IP (Standard TCP/IP Port).
Once I do that, printing works great. It must be something with the iPrint
client. Not all users sharing the same printer have this issue and none of
the users having this problem seem to have unique applications. The other
strange thing is that a user that has printed fine for months could start
having the problem tomorrow.

Any help will be greatly appreciated.