Trying to install Drupal 7 on Mac OS X, I am given only the option of a SQLite database, and not MySQL. In terms of usability this is confusing, as I just presumed that the fields I was being given were for MySQL connection information, and only realised that assumption was incorrect when I got SQLite errors:
Failed to create a test table on your SQLite database server with the command CREATE TABLE drupal_install_test (id int NULL). SQLite reports the following message: SQLSTATE[HY000]  unable to open database file.
PDO::getAvailableDrivers() is returning only sqlite as being available.
I had thought this was a localised problem because of the way my environment is set up, but it seems from this support thread and blog post that it is a wider problem with the standard installation of PDO on Mac OS X 5 (Leopard), so others are likely to run into it too.
At a minimum we should highlight the database type that is being used (the only mention is in the help text at the moment), even if there is only one available, but beyond that, perhaps find a workaround that could be coded in (MySQL still works, just not through PDO) or provide documentation that would help developers fix the issue on their system.