hi,

the 11.059MHz is used especially with standard RS232 speeds in mind (the dividers allow you to reach many default speeds, up to 56200 baud)

I would say "up to 115200 (and even 345600) if timer 2 is used for baud rate clock"

Here is baud rate calculator: http://www.keil.com/c51/baudrate.asp

Regards,
Oleg