A pin hole leak developed in a horizontal cast iron steam pipe, about 3/4 inch pipe I believe. Can this be replaced easily with a section of PVC??

Don't think so. First, because I doubt that you have cast iron in 3/4". Probably steel. And second, because PVC would not with stand the temperature. You can't even use PVC for hot water much less steam.

Harold is correct! Steam pipe will be black steel pipe and you can't use PVC pipe.

Here, the only real options will depend on the condition of the pipe. If the pipe is in great condition then a plumber could probably come in and cut out a 1 foot section of the pipe and cut new threads on the pipe and reconnect using a coupling, a pipe nipple and a union.

Another option would be to cut out the pipe back to the nearest fittings, remove the pipe ends from the threaded fittings, and then install copper pipe in between the fittings... could also use steel pipe with a union.

A "quick fix" would be to get some rainbow packing material and a clamp and just fix the leak temporarily. This fix will work but will not last long... ;) Rainbow packing is a gasket making material that comes in sheet form... only available at a local plumbing supply store (sold in 1 foot increments).

Good luck!

Mark