Transparency in design is a very good thing. This is my opinion on the subject - what I've seen so far of the release is impressive technically, however the PHP userland API design is something out of late 1999.
You're not going to find people willing to backport code to use Microsoft SQL Server for their old PHP4 code - very "popular" (i.e. many downloads and installs) software is mainly php4 and not necessarily the most used. You probably should look at number of users and average load vs. number of downloads and installs - yahoo is probably a better indicator of good code than a 10 year old with an anime forum using phpbb. (Oh, and by the way 4.x is End of Lifed) I believe instead you need to be attracting the people writing the new PHP5 stuff.
If you want to see how user land php api's work take a look at Zend Framework, Symphony, Solar, even CakePHP although it is still 4 compatible. Propel and Doctrine are the big PHP5 ORM layers, look at PDO and the Mysqli extension's OO support. Also look at the new DB2 driver IBM is writing or the Oci8 extension. This is where you can see how new PHP internal API's are structured.
Actually I would really encourage you to get a PDO driver out. The push right now in PHP C land is to gradually abandon the individual drivers in favor of PDO. For example, you could harness the streams work you did with the PDO blob support.
To quote Rasmus Lerdorf
"PDO isn't database abstraction. It is internal API
abstraction. All the common things a PHP database extension needs to do
internally in PHP is handled nicely by PDO making it much easier to
write a new database extension under PDO. The fact that you get a bit
of userspace API abstraction as well is just a side effect."
What does this mean for people wanting to move from Mysql or the old Mssql dblib based driver? Well if they're already using PDO the changes needed are minimal, and PHP developers are generally pretty lazy. So make it easy for us, and please have an API that is modern.