Safari users can't create content

Issue Summary

Steps to reproduce:
1. Using Safari 3.2.1 Mac, log in to site with any account that has create content permissions.
2. Click Create Page (or any other content type)
3. Fill in any required fields.
4. Click Save or Preview.

Expectation: Page is saved.

Actual result: Page refreshes with no message displayed. Values entered in step 3 are still in place. No indication in Watchdog table or Reports admin that I've done anything except login.

Other information:

• Interestingly, if I leave a required field blank, I do get the expected validation message.
• Other forms on the site (e.g. node edit, contact, and webforms) are behaving as expected.
• Add node form works as expected in IE7 Win, FF Win, and FF Mac.
• Tried resetting Safari, clearing all Drupal caches, resetting permissions, and clearing the theme registry.

Drupal 6.13
MySQL database 5.0.67
PHP 5.2.9
Web server Apache
Web browser Safari 3.2.1
Site is based on Blueprint theme.

All the best,
Dave