I am in a process of:
1/ moving configuration checks from install.php into environment - new section PHP_SETTINGS in environment.xml and setup.php - why?? because many ppl are reporting problems when configuration changes

• critical PHP settings are now checked on each page see lbi/setuplib.php function setup_validate_php_configuration() - magic_quotes are deadly for security reasons, session auto start breaks sessions completely and magic quotes runtime break everything too
• less critical and recommended settings were moved into environment.xml - these things need not be installer at all, users should be remonded during each upgrade that something is wrong or not optimal
• test php version and curl ext and show error only if it fails - this way most ppl will see only environment tests from admin/index.php instead of incomplete two tests in current installer

2/ moving environment test from install.php to admin/index.php - problems here are two - first all strings in environment test need to go into install lang packs, when creating config.php manually the environment tests were not executed at all (critical)

3/ installation strings are getting bigger and bigger - seems easier to download installer right after successful directory selection; this way we can use normal string handling in db selection/configuration/testing etc., this way we will need only some 15 strings instead of 50+ in install/en_utf8/installer.php

4/ enabling full debug and error dispaly in installer + tweaking all strings - this could prevent very many installer bug reports finally