One option is the DS2480 1-wire-to-serial IC:
**broken link removed**
.. another option is the PICAXE microcontroller with it's 1-wire ready-to-go BASIC commands:
1-wire Serial No: "readowsn"
1-wire Clock: "readowclk"
etc. etc.
Sorry, I do not see any easy solution to this. I do not like the approach, either. If I were to design something like this, I would probably use a CMOS gate with large series resistors on the inputs and connect that to the 18S20.
I think you will need the Schottky diodes if you are going to use the schematic as is, since they provide protection for the 18S20.
So, what's happening if by mistake one of the DS18B20 pins will make a contact with the earth ? (the PC GND is connected to the protective earth).
There is no limiting current resistor on the RS232 output, so all RS232 lines involved will use the self limiting feature of the internal TTL-RS232 converter, from the PC which is quite wrong...