Does Wireshark support Modbus/TCP?

Can I use WireShark as a sniffer on Modbus/TCP traffic?

Wireshark includes, at least in the top of the SVN trunk, a Modbus/TCP dissector:

$ tshark -G protocols | egrep -i modbus
$ CIP Modbus Object CIPMB   cipmb
$ Modbus    Modbus  modbus
$ Modbus/TCP    Modbus/TCP  mbtcp

and I think that was added before the 1.6 release, so 1.6.x, and possibly earlier releases, should have it, so you should be able to decode and analyzer Modbus/TCP traffic on any network on which Wireshark can capture traffic.

Can I use it for simulating either a Modbus/TCP client or a server?

No. Wireshark is a packet sniffer (network analyzer), not a network simulator.