as I have a MacBook from my office, and a MacBook Pro for professional
(and private) use, I was able to test PureBasic with Snow Leopard.
As of know, I cannot recommend using Snow Leopard for coding with PureBasic.
So far, I have installed PureBasic 4.31/x86 with xCode for Leopard on the
MacBook. With this, I am able to produce code which runs great on Leopard.
My private MacBook Pro runs PB 4.31/x86 with xCode for Snow Leopard.
Also, this works great without any problems so far (testing for one week now)
Not possible at the moment is to run Leopard-PB-Applications on Snow Leopard
and vice versa. I am lucky because of the second MacBook, but sad because
I can't code on one machine only

Workaround-Tip: install Mac OS X Snow Leopard on your internal harddrive
(works great, this OS is definetely a recommandation!!!) Then install Mac OS X
Leopard on an external harddrive or USB-Stick with at least 16gb FREE. There
you install xCode for Leopard and put everything on what you need for
PureBasicing. You may exchange Data between Leopard and Snow Leopard,
as you are able to access the directories directly... Sadly you have to reboot
when testing and compiling for the other OS.
Hope this will be fixed soon with PureBasic and/or xCode.
p.s. I didn't try to install the older xCode for Leopard atm due to lack of time.