The way textarea.js rewrites the DOM triggers rendering bugs in IE/Win 6 (at least) on themes where any ancestor of a resizable textarea has margins. This includes, among others, Garland in Drupal 5. The problem occurs because textarea.js reparents the textarea field to a new div that has an explicit width assigned, granting hasLayout and triggering the bug.
See http://jaspan.com/ie-inherited-margin-bug-form-elements-and-haslayout for a discussion.
Patch for Drupal 5 attached.