LocalTalk and 10.1

Appletalk works well in 10.1.
File sharing over IP works well in 10.1.

What exactly, in the realm of networking, are you wanting to do?
Is the laserwriter on a network connected with ethernet cables or phone net or something else? Directly connected to the computer? If the laserwriter has an ethernet connection then a direct ethernet to ethernet cable works or if you have cable modem or DSL you'd need a hub to get both machines on the network together.
If the printer talks Appletalk then the OS X machine should find it (with 10.1, the appletalk networking in 10.0.x sucked) with no problem. It's more a matter of physical connections than anything else.
I would like to use my LaserWriter 4/600 PS via the serial port, like under Mac OS 9.1! The LaserWriter has no other Port available.

In Mac OS X 10.0.4 LocalTalk thru the serial port was not included. :(

Is LocalTalk thru the serial port in 10.1 included?

Is it in schedule for future versions of Mac OS X?

Or do you know a third party solution?

I doubt very seriously that LocalTalk through a serial port will ever be supported on OS X.

Take a look at the Farallon EtherMac iPrint adapters.