Simply, there's a fatal part error in the template when short tags are enabled.
As reported in http://drupal.org/coding-standards#phptags, short tags should never be used with Drupal; I take that means they should never be enabled with Drupal.
I would say they are not mutually exclusive. And its not a big deal to support them.
This is sort of a critical but trivial bug fix I'd like to see committed. The patch even includes some additional documentation to match the sort of de facto template documentation.
Just a note, this is similar to the approach taken by core's rss.xml handler, views' RSS template, fivestar's AJAX callback and views bonus' xml template. So its pretty common approach to addressing this sort of problem.
The code has been changed, and committed.
Thanks for your report.
I forgot to change status.
Automatically closed -- issue fixed for 2 weeks with no activity.
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