On at least two machines running Solaris 10 Update 1, no /dev/ttyb is created even though the device is present. This is because Solaris uses ACPI to determine what serial ports are present and on these machines, COM2 is not listed in the ACPI table. I'm opening this bug because Suse Linux Enterprise Server 9 creates a /dev/ttyS1 on these machines.
The machines are the IBM BladeCenter HS40 and the HS20 model 7981. The explanation from IBM is that COM2 is only used for Serial Over LAN and isn't available for general use. IBM has since acknowleged that it is a bug in the BIOS and the next BIOS upgrade for the HS20 model 7981 will include COM2 in the ACPI tables. There is no schedule for changing the HS40 BIOS.
Work Around
How to do this is documented here:
http://docs.sun.com/app/docs/doc/819-5455/6n7hnfnh3?q=BladeCenter&a=view