Details
Bug
Major
Description
When "PHPBB_USE_BOARD_URL_PATH" is defined as true so the web paths are all absolute, the link to the stylesheet will still not be an absolute link due to the user style storing the theme in the database.
Fix is to change the $phpbb_root_path used in the T_STYLESHEET_LINK tempalte variable to $web_path.
Activity
Yes, left it out because it calls style.php - is it causing problems for you specifically?
Yes it was causing problems for me because when I start using the phpBB as a framework and have rewrite urls, the paths will be incorrect unless the style link is absolute.
Why did you leave it out because it calls style.php?
Because we do not use the constant for php files... for example linking to ucp.php 
ucp.php is passed through append_sid(), as well as style.php. If the constant is used append_sid() is not utilised.
Ah, well the reason I am having problems with this is because I am using mod rewrite for urls. Since the urls can be very deep, absolute paths will not work which makes the style all messed up. The simple fix for this is to use the web path which gets defined if you use the PHPBB_USE_BOARD_URL_PATH constant, however since the dynamic style sheet is sitll using the absolute path, you may get unexpected results if the stylesheet is parsed by style.php
As 3.1 has no style.php this should be effectively fixed.
As far as i remember i left this out intentionally - but i will have a look.