I was recently contacted twice for more information on this topic, please find below my responses. As well, feel free to post your comments and input on this topic.
What is an Instructional Systems Designer?
The definition will differ but, instead of planning only training initiatives. You would plan software or application role outs. Sometimes this is called Technical Process Engineering, the debt of involvement will depend on your experience and skills level.
Is it similar to a Instructional Designer?
Not always, some people are not able to crossover. My opinion is you need a good technical background and strong instructional design background. Sort of like some architects design bridges others towers. Real experienced ones can design just about anything, even concept items never thought of.