Forum OpenACS Q&A: Response to How about a port of ACS to PHP or python?
Lots, and lots of time that most people would see as wasted effort given the efforts necessary to bring ACS 4.0 to PG.
If I say too much here its only going to be going over previous threads which thrash out this subject en mass. I for one would like to see (Open)ACS written in PHP given the larger install base of Apache, but given you can now get mod_aolserver its starting to discount this line. Many man hours(years?) have been put into ACS and starting the whole process over again would be painful.
And yep, threading has been an issue (apparently being worked on) and still causes problems when you try and use it with aolserver or IIS.