Speaking of Lantronix checkout their X-PORT product.
If you you do not want to do the minimal glue electronics, order 2 development boards (The boards are well done, compact, and very friendly). You can have point to point serial over ethernet using two boards with them simply setup in serial modem mode. One on the PLC taking serial and passing directly to ethernet (IP based), the other board converting the ethernet back to serial at the other end either on your PC or another PLC.
Or use one board on the PLC and use an utility they give for free that converts your ethernet traffic from the xport to serial type traffic on the PC end to look like it is actually coming in on a com port (virtual port). Great for com port based software like tlserver.
I have not tried it, but I believe it should work!
The X-PORT also has three discrete IO lines, which you could tie to the PLC IO. Since the X-PORT can send email, imagine just flipping an output on the PLC to send an alarm via email.
The X-PORT also has many other features, which might enable you to built a mini web based scada front end for your PLC.
Tim,
If you are looking for an Ethernet to serial converter look into:
http://www.lantronix.com/index.html
Their UDS-10 device server is around $75 and I have used it very successfully with TL server and the T100MD series PLC's