Does this only affect "search.module"? ie, could a site use Drupal 4.6 on pre-PHP 4.3.3 if they disable the search module? (I realize this isn't the point, I'm just wanting to understand...)
Yes, I am not aware of any problems with the current minimum PHP requirement (4.1.0?) other than search.module. By the way I did some more testing, and it seems that the pre-4.3.3 UTF-8 support was more broken than I thought: the only option is in fact a byte-based regular expression, as even using ISO-8859-1 characters is problematic with anything other than simple character matches. So it would be a solution where not only it is broken on anything non-western-european, but it would be very difficult to make this patch in the first place. Steven