Yosemite + PCI Serial Card? I have a PCIe serial card in a Mac Pro based on the 16950 that's been working fine up until Yosemite. It's been working with the built-in Apple16x50Serial.kext driver.

With Yosemite, it looks like the driver loads okay, but no serial port appears under /dev/, and no IOBSDSerialClient for the card appears in ioreg either.

Anyone else have the same problem please let me know, or if you have a solution even better. :-)

Thanks
Are you sure the driver loads fine? Since Yosemite kexts need to be signed. Check the kextstats man page for the status of your kexts. 

Check out the TrimEnabler FAQ -- they have quite some information on kext signing and the troubles you might experience due to this.